diff --git a/docs/usage.md b/docs/usage.md
index 70aaf8b0a..279ed7cf7 100644
--- a/docs/usage.md
+++ b/docs/usage.md
@@ -100,6 +100,7 @@ Think of versions as **file history** for a document.
 - In document detail, selecting a version switches the preview, file metadata and content (and download etc buttons) to that version.
 - Deleting a non-root version keeps metadata and falls back to the latest remaining version.
 - From the document list, select two or more documents and choose **Merge as versions** to combine them under one entry. Select the root document whose metadata and permissions should be retained; the other selected documents become file versions. The root may already have versions, but documents being added as versions must not have version histories of their own.
+- From a document's **Versions** menu, choose **Existing** to search for another document and add it as a version of the current document.
 
 ### Management Lists
 
diff --git a/src-ui/src/app/components/document-detail/document-version-dropdown/add-existing-document-version-dialog/add-existing-document-version-dialog.component.html b/src-ui/src/app/components/document-detail/document-version-dropdown/add-existing-document-version-dialog/add-existing-document-version-dialog.component.html
new file mode 100644
index 000000000..c92993c93
--- /dev/null
+++ b/src-ui/src/app/components/document-detail/document-version-dropdown/add-existing-document-version-dialog/add-existing-document-version-dialog.component.html
@@ -0,0 +1,18 @@
+<div class="modal-header">
+  <h4 class="modal-title" i18n>Add existing document as version</h4>
+  <button type="button" class="btn-close" aria-label="Close" (click)="cancel()"></button>
+</div>
+<div class="modal-body">
+  <pngx-input-document-link
+    [(ngModel)]="selectedDocumentIDs"
+    [parentDocumentID]="rootDocumentID"
+    [minimal]="true"
+    placeholder="Search for a document"
+    i18n-placeholder
+  ></pngx-input-document-link>
+  <div class="form-text mt-2" i18n>Select one document to add as a version.</div>
+</div>
+<div class="modal-footer">
+  <button type="button" class="btn btn-outline-secondary" (click)="cancel()" [disabled]="!buttonsEnabled" i18n>Cancel</button>
+  <button type="button" class="btn btn-primary" (click)="confirm()" [disabled]="!buttonsEnabled || selectedDocumentIDs.length !== 1" i18n>Add version</button>
+</div>
diff --git a/src-ui/src/app/components/document-detail/document-version-dropdown/add-existing-document-version-dialog/add-existing-document-version-dialog.component.spec.ts b/src-ui/src/app/components/document-detail/document-version-dropdown/add-existing-document-version-dialog/add-existing-document-version-dialog.component.spec.ts
new file mode 100644
index 000000000..8d8a20a2a
--- /dev/null
+++ b/src-ui/src/app/components/document-detail/document-version-dropdown/add-existing-document-version-dialog/add-existing-document-version-dialog.component.spec.ts
@@ -0,0 +1,56 @@
+import { ComponentFixture, TestBed } from '@angular/core/testing'
+import { NgbActiveModal } from '@ng-bootstrap/ng-bootstrap'
+import { DocumentService } from 'src/app/services/rest/document.service'
+import { AddExistingDocumentVersionDialogComponent } from './add-existing-document-version-dialog.component'
+
+describe('AddExistingDocumentVersionDialogComponent', () => {
+  let component: AddExistingDocumentVersionDialogComponent
+  let fixture: ComponentFixture<AddExistingDocumentVersionDialogComponent>
+  let activeModal: jest.Mocked<Pick<NgbActiveModal, 'dismiss'>>
+
+  beforeEach(async () => {
+    activeModal = { dismiss: jest.fn() }
+    await TestBed.configureTestingModule({
+      imports: [AddExistingDocumentVersionDialogComponent],
+      providers: [
+        {
+          provide: NgbActiveModal,
+          useValue: activeModal,
+        },
+        {
+          provide: DocumentService,
+          useValue: {},
+        },
+      ],
+    }).compileComponents()
+
+    fixture = TestBed.createComponent(AddExistingDocumentVersionDialogComponent)
+    component = fixture.componentInstance
+    component.rootDocumentID = 3
+    fixture.detectChanges()
+  })
+
+  it('should emit the single selected document', () => {
+    const emitSpy = jest.spyOn(component.confirmClicked, 'emit')
+    component.selectedDocumentIDs = [20]
+
+    component.confirm()
+
+    expect(emitSpy).toHaveBeenCalledWith(20)
+  })
+
+  it('should require exactly one selected document', () => {
+    const emitSpy = jest.spyOn(component.confirmClicked, 'emit')
+    component.selectedDocumentIDs = [20, 21]
+
+    component.confirm()
+
+    expect(emitSpy).not.toHaveBeenCalled()
+  })
+
+  it('should dismiss on cancel', () => {
+    component.cancel()
+
+    expect(activeModal.dismiss).toHaveBeenCalled()
+  })
+})
diff --git a/src-ui/src/app/components/document-detail/document-version-dropdown/add-existing-document-version-dialog/add-existing-document-version-dialog.component.ts b/src-ui/src/app/components/document-detail/document-version-dropdown/add-existing-document-version-dialog/add-existing-document-version-dialog.component.ts
new file mode 100644
index 000000000..ce967a4ad
--- /dev/null
+++ b/src-ui/src/app/components/document-detail/document-version-dropdown/add-existing-document-version-dialog/add-existing-document-version-dialog.component.ts
@@ -0,0 +1,28 @@
+import { Component, EventEmitter, Input, Output, inject } from '@angular/core'
+import { FormsModule } from '@angular/forms'
+import { NgbActiveModal } from '@ng-bootstrap/ng-bootstrap'
+import { DocumentLinkComponent } from 'src/app/components/common/input/document-link/document-link.component'
+
+@Component({
+  selector: 'pngx-add-existing-document-version-dialog',
+  templateUrl: './add-existing-document-version-dialog.component.html',
+  imports: [DocumentLinkComponent, FormsModule],
+})
+export class AddExistingDocumentVersionDialogComponent {
+  private readonly activeModal = inject(NgbActiveModal)
+
+  @Input() rootDocumentID: number
+  @Output() confirmClicked = new EventEmitter<number>()
+
+  selectedDocumentIDs: number[] = []
+  buttonsEnabled = true
+
+  confirm(): void {
+    if (this.selectedDocumentIDs.length !== 1) return
+    this.confirmClicked.emit(this.selectedDocumentIDs[0])
+  }
+
+  cancel(): void {
+    this.activeModal.dismiss()
+  }
+}
diff --git a/src-ui/src/app/components/document-detail/document-version-dropdown/document-version-dropdown.component.html b/src-ui/src/app/components/document-detail/document-version-dropdown/document-version-dropdown.component.html
index b37134618..9032a2e4a 100644
--- a/src-ui/src/app/components/document-detail/document-version-dropdown/document-version-dropdown.component.html
+++ b/src-ui/src/app/components/document-detail/document-version-dropdown/document-version-dropdown.component.html
@@ -24,13 +24,26 @@
           class="visually-hidden"
           (change)="onVersionFileSelected($event)"
         />
-        <button
-          class="btn btn-sm btn-outline-secondary w-100"
-          (click)="versionFileInput.click()"
-          [disabled]="!userIsOwner || !userCanEdit"
-        >
-          <i-bs name="file-earmark-plus"></i-bs><span class="ps-1" i18n>Add new version</span>
-        </button>
+        <div class="btn-group btn-group-sm w-100">
+          <button
+            class="btn btn-sm btn-outline-secondary w-100"
+            (click)="versionFileInput.click()"
+            [disabled]="!userIsOwner || !userCanEdit"
+            title="Upload a new version"
+            i18n-title
+          >
+            <i-bs name="file-earmark-plus"></i-bs><span class="ps-1" i18n>Upload</span>
+          </button>
+          <button
+            class="btn btn-sm btn-outline-secondary w-100"
+            (click)="addExistingDocumentAsVersion()"
+            [disabled]="!userIsOwner || !userCanEdit"
+            title="Use an existing document"
+            i18n-title
+          >
+            <i-bs name="file-earmark"></i-bs><span class="ps-1" i18n>Existing</span>
+          </button>
+        </div>
       } @else {
         @switch (versionUploadState()) {
           @case (UploadState.Uploading) {
diff --git a/src-ui/src/app/components/document-detail/document-version-dropdown/document-version-dropdown.component.spec.ts b/src-ui/src/app/components/document-detail/document-version-dropdown/document-version-dropdown.component.spec.ts
index b604b3738..9ec9b4e63 100644
--- a/src-ui/src/app/components/document-detail/document-version-dropdown/document-version-dropdown.component.spec.ts
+++ b/src-ui/src/app/components/document-detail/document-version-dropdown/document-version-dropdown.component.spec.ts
@@ -1,6 +1,7 @@
 import { DatePipe } from '@angular/common'
 import { SimpleChange } from '@angular/core'
 import { ComponentFixture, TestBed } from '@angular/core/testing'
+import { NgbModal } from '@ng-bootstrap/ng-bootstrap'
 import { NgxBootstrapIconsModule, allIcons } from 'ngx-bootstrap-icons'
 import { Subject, of, throwError } from 'rxjs'
 import { DocumentVersionInfo } from 'src/app/data/document'
@@ -19,12 +20,17 @@ describe('DocumentVersionDropdownComponent', () => {
   let documentService: jest.Mocked<
     Pick<
       DocumentService,
-      'deleteVersion' | 'getVersions' | 'uploadVersion' | 'updateVersionLabel'
+      | 'deleteVersion'
+      | 'getVersions'
+      | 'mergeDocumentsAsVersions'
+      | 'uploadVersion'
+      | 'updateVersionLabel'
     >
   >
   let toastService: jest.Mocked<Pick<ToastService, 'showError' | 'showInfo'>>
   let finished$: Subject<{ taskId: string }>
   let failed$: Subject<{ taskId: string; message?: string }>
+  let modalService: jest.Mocked<Pick<NgbModal, 'open'>>
 
   beforeEach(async () => {
     finished$ = new Subject<{ taskId: string }>()
@@ -32,9 +38,11 @@ describe('DocumentVersionDropdownComponent', () => {
     documentService = {
       deleteVersion: jest.fn(),
       getVersions: jest.fn(),
+      mergeDocumentsAsVersions: jest.fn(),
       uploadVersion: jest.fn(),
       updateVersionLabel: jest.fn(),
     }
+    modalService = { open: jest.fn() }
     toastService = {
       showError: jest.fn(),
       showInfo: jest.fn(),
@@ -61,6 +69,10 @@ describe('DocumentVersionDropdownComponent', () => {
           provide: ToastService,
           useValue: toastService,
         },
+        {
+          provide: NgbModal,
+          useValue: modalService,
+        },
         {
           provide: WebsocketStatusService,
           useValue: {
@@ -323,4 +335,43 @@ describe('DocumentVersionDropdownComponent', () => {
     expect(component.editingVersionId).toBeNull()
     expect(component.versionLabelDraft).toEqual('')
   })
+
+  it('addExistingDocumentAsVersion should merge with a label and refresh versions', () => {
+    const confirmClicked = new Subject<number>()
+    const modal = {
+      componentInstance: {
+        rootDocumentID: null,
+        buttonsEnabled: true,
+        confirmClicked,
+      },
+      close: jest.fn(),
+    }
+    modalService.open.mockReturnValue(modal as any)
+    documentService.mergeDocumentsAsVersions.mockReturnValue(of({} as any))
+    const versions: DocumentVersionInfo[] = [
+      { id: 3, is_root: true, checksum: 'aaaa' },
+      { id: 20, is_root: false, checksum: 'cccc' },
+    ]
+    documentService.getVersions.mockReturnValue(of({ id: 3, versions } as any))
+    component.newVersionLabel = '  Imported  '
+    const versionsEmitSpy = jest.spyOn(component.versionsUpdated, 'emit')
+    const selectedEmitSpy = jest.spyOn(component.versionSelected, 'emit')
+
+    component.addExistingDocumentAsVersion()
+    expect(modal.componentInstance.rootDocumentID).toEqual(3)
+    confirmClicked.next(20)
+
+    expect(documentService.mergeDocumentsAsVersions).toHaveBeenCalledWith(
+      [3, 20],
+      3,
+      'Imported'
+    )
+    expect(documentService.updateVersionLabel).not.toHaveBeenCalled()
+    expect(documentService.getVersions).toHaveBeenCalledWith(3)
+    expect(versionsEmitSpy).toHaveBeenCalledWith(versions)
+    expect(selectedEmitSpy).toHaveBeenCalledWith(20)
+    expect(component.newVersionLabel).toEqual('')
+    expect(modal.close).toHaveBeenCalled()
+    expect(toastService.showInfo).toHaveBeenCalled()
+  })
 })
diff --git a/src-ui/src/app/components/document-detail/document-version-dropdown/document-version-dropdown.component.ts b/src-ui/src/app/components/document-detail/document-version-dropdown/document-version-dropdown.component.ts
index d5b9052e2..236236948 100644
--- a/src-ui/src/app/components/document-detail/document-version-dropdown/document-version-dropdown.component.ts
+++ b/src-ui/src/app/components/document-detail/document-version-dropdown/document-version-dropdown.component.ts
@@ -11,7 +11,7 @@ import {
   SimpleChanges,
 } from '@angular/core'
 import { FormsModule } from '@angular/forms'
-import { NgbDropdownModule } from '@ng-bootstrap/ng-bootstrap'
+import { NgbDropdownModule, NgbModal } from '@ng-bootstrap/ng-bootstrap'
 import { NgxBootstrapIconsModule } from 'ngx-bootstrap-icons'
 import { merge, of, Subject } from 'rxjs'
 import {
@@ -33,6 +33,7 @@ import {
   WebsocketStatusService,
 } from 'src/app/services/websocket-status.service'
 import { ConfirmButtonComponent } from '../../common/confirm-button/confirm-button.component'
+import { AddExistingDocumentVersionDialogComponent } from './add-existing-document-version-dialog/add-existing-document-version-dialog.component'
 
 @Component({
   selector: 'pngx-document-version-dropdown',
@@ -69,6 +70,7 @@ export class DocumentVersionDropdownComponent implements OnChanges, OnDestroy {
   private readonly documentsService = inject(DocumentService)
   private readonly toastService = inject(ToastService)
   private readonly websocketStatusService = inject(WebsocketStatusService)
+  private readonly modalService = inject(NgbModal)
   private readonly destroy$ = new Subject<void>()
   private readonly documentChange$ = new Subject<void>()
 
@@ -278,6 +280,56 @@ export class DocumentVersionDropdownComponent implements OnChanges, OnDestroy {
       })
   }
 
+  addExistingDocumentAsVersion(): void {
+    const modal = this.modalService.open(
+      AddExistingDocumentVersionDialogComponent,
+      { backdrop: 'static' }
+    )
+    const dialog =
+      modal.componentInstance as AddExistingDocumentVersionDialogComponent
+    dialog.rootDocumentID = this.documentId
+    dialog.confirmClicked
+      .pipe(takeUntil(this.destroy$), takeUntil(this.documentChange$))
+      .subscribe((existingDocumentID) => {
+        dialog.buttonsEnabled = false
+        const versionLabel = this.newVersionLabel?.trim()
+        this.documentsService
+          .mergeDocumentsAsVersions(
+            [this.documentId, existingDocumentID],
+            this.documentId,
+            versionLabel
+          )
+          .pipe(
+            switchMap(() => this.documentsService.getVersions(this.documentId)),
+            first(),
+            finalize(() => (dialog.buttonsEnabled = true)),
+            takeUntil(this.destroy$),
+            takeUntil(this.documentChange$)
+          )
+          .subscribe({
+            next: (document) => {
+              if (document?.versions) {
+                this.versionsUpdated.emit(document.versions)
+                this.versionSelected.emit(
+                  Math.max(...document.versions.map((version) => version.id))
+                )
+              }
+              this.newVersionLabel = ''
+              modal.close()
+              this.toastService.showInfo(
+                $localize`Existing document added as a version.`
+              )
+            },
+            error: (error) => {
+              this.toastService.showError(
+                $localize`Error adding existing document as a version`,
+                error
+              )
+            },
+          })
+      })
+  }
+
   clearVersionUploadStatus(): void {
     this.versionUploadState.set(UploadState.Idle)
     this.versionUploadError.set(null)