diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index 2cb4ecf..0162c48 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -2233,7 +2233,7 @@
 link:#applypatchpatchset-input[ApplyPatchPatchSetInput] entity.
 
 If a base commit is given, the patch is applied on top of it. Otherwise, the
-patch is applied on top of the target change's branch tip.
+patch is applied on top of the target change's original parent.
 
 Applying the patch will fail if the destination change is closed, or in case of any conflicts.
 
diff --git a/java/com/google/gerrit/server/restapi/change/ApplyPatch.java b/java/com/google/gerrit/server/restapi/change/ApplyPatch.java
index c6e1e5e..f368eb9 100644
--- a/java/com/google/gerrit/server/restapi/change/ApplyPatch.java
+++ b/java/com/google/gerrit/server/restapi/change/ApplyPatch.java
@@ -16,6 +16,7 @@
 
 import static com.google.gerrit.server.update.context.RefUpdateContext.RefUpdateType.CHANGE_MODIFICATION;
 
+import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableList;
 import com.google.gerrit.entities.BranchNameKey;
 import com.google.gerrit.entities.Change;
@@ -24,6 +25,7 @@
 import com.google.gerrit.extensions.api.changes.ApplyPatchPatchSetInput;
 import com.google.gerrit.extensions.client.ListChangesOption;
 import com.google.gerrit.extensions.common.ChangeInfo;
+import com.google.gerrit.extensions.restapi.BadRequestException;
 import com.google.gerrit.extensions.restapi.PreconditionFailedException;
 import com.google.gerrit.extensions.restapi.ResourceConflictException;
 import com.google.gerrit.extensions.restapi.ResourceNotFoundException;
@@ -139,9 +141,21 @@
                 destChange.change().getStatus().name()));
       }
 
-      RevCommit baseCommit =
-          CommitUtil.getBaseCommit(
-              project.get(), queryProvider.get(), revWalk, destRef, input.base);
+      RevCommit baseCommit;
+      if (!Strings.isNullOrEmpty(input.base)) {
+        baseCommit =
+            CommitUtil.getBaseCommit(
+                project.get(), queryProvider.get(), revWalk, destRef, input.base);
+      } else {
+        RevCommit latestPatchset = revWalk.parseCommit(destChange.currentPatchSet().commitId());
+        if (latestPatchset.getParentCount() != 1) {
+          throw new BadRequestException(
+              String.format(
+                  "Cannot parse base commit for a change with none or multiple parents. Change ID: %s.",
+                  destChange.getId()));
+        }
+        baseCommit = revWalk.parseCommit(latestPatchset.getParent(0));
+      }
       ObjectId treeId = ApplyPatchUtil.applyPatch(repo, oi, input.patch, baseCommit);
 
       Instant now = TimeUtil.now();
diff --git a/javatests/com/google/gerrit/acceptance/api/change/ApplyPatchIT.java b/javatests/com/google/gerrit/acceptance/api/change/ApplyPatchIT.java
index a1b9e3a..b451656 100644
--- a/javatests/com/google/gerrit/acceptance/api/change/ApplyPatchIT.java
+++ b/javatests/com/google/gerrit/acceptance/api/change/ApplyPatchIT.java
@@ -17,6 +17,9 @@
 import static com.google.common.truth.Truth.assertThat;
 import static com.google.gerrit.acceptance.testsuite.project.TestProjectUpdate.block;
 import static com.google.gerrit.acceptance.testsuite.project.TestProjectUpdate.permissionKey;
+import static com.google.gerrit.extensions.client.ListChangesOption.CURRENT_COMMIT;
+import static com.google.gerrit.extensions.client.ListChangesOption.CURRENT_FILES;
+import static com.google.gerrit.extensions.client.ListChangesOption.CURRENT_REVISION;
 import static com.google.gerrit.server.group.SystemGroupBackend.REGISTERED_USERS;
 import static com.google.gerrit.testing.GerritJUnit.assertThrows;
 import static java.nio.charset.StandardCharsets.UTF_8;
@@ -47,6 +50,7 @@
 import com.google.inject.Inject;
 import java.io.IOException;
 import org.eclipse.jgit.api.errors.PatchApplyException;
+import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.util.Base64;
 import org.junit.Test;
 
@@ -167,7 +171,7 @@
     ChangeInfo result = applyPatch(in);
 
     BinaryResult resultPatch = gApi.changes().id(result.id).current().patch();
-    assertThat(removeHeader(resultPatch)).isEqualTo(removeHeader(originalPatch));
+    assertThat(cleanPatch(resultPatch)).isEqualTo(cleanPatch(originalPatch));
   }
 
   @Test
@@ -189,52 +193,54 @@
     ChangeInfo result = applyPatch(in);
 
     BinaryResult resultPatch = gApi.changes().id(result.id).current().patch();
-    assertThat(removeHeader(resultPatch)).isEqualTo(removeHeader(originalPatch));
+    assertThat(cleanPatch(resultPatch)).isEqualTo(cleanPatch(originalPatch));
   }
 
   @Test
   public void applyGerritBasedPatchUsingRest_success() throws Exception {
     String head = getHead(repo(), HEAD).name();
-    createBranchWithRevision(BranchNameKey.create(project, "branch"), head);
-    PushOneCommit.Result baseCommit = createChange("Add file", ADDED_FILE_NAME, ADDED_FILE_CONTENT);
-    baseCommit.assertOkStatus();
     createBranchWithRevision(BranchNameKey.create(project, DESTINATION_BRANCH), head);
+    PushOneCommit.Result destChange = createChange("refs/for/" + DESTINATION_BRANCH);
+    createBranchWithRevision(BranchNameKey.create(project, "branch"), head);
+    PushOneCommit.Result baseCommit =
+        createChange(testRepo, "branch", "Add file", ADDED_FILE_NAME, ADDED_FILE_CONTENT, "");
+    baseCommit.assertOkStatus();
     RestResponse patchResp =
         userRestSession.get("/changes/" + baseCommit.getChangeId() + "/revisions/current/patch");
     patchResp.assertOK();
     String originalPatch = new String(Base64.decode(patchResp.getEntityContent()), UTF_8);
     ApplyPatchPatchSetInput in = buildInput(originalPatch);
-    PushOneCommit.Result destChange = createChange();
 
     RestResponse resp =
         adminRestSession.post("/changes/" + destChange.getChangeId() + "/patch:apply", in);
 
     resp.assertOK();
     BinaryResult resultPatch = gApi.changes().id(destChange.getChangeId()).current().patch();
-    assertThat(removeHeader(resultPatch)).isEqualTo(removeHeader(originalPatch));
+    assertThat(cleanPatch(resultPatch)).isEqualTo(cleanPatch(originalPatch));
   }
 
   @Test
   public void applyGerritBasedPatchUsingRestWithEncodedPatch_success() throws Exception {
     String head = getHead(repo(), HEAD).name();
-    createBranchWithRevision(BranchNameKey.create(project, "branch"), head);
-    PushOneCommit.Result baseCommit = createChange("Add file", ADDED_FILE_NAME, ADDED_FILE_CONTENT);
-    baseCommit.assertOkStatus();
     createBranchWithRevision(BranchNameKey.create(project, DESTINATION_BRANCH), head);
+    PushOneCommit.Result destChange = createChange("refs/for/" + DESTINATION_BRANCH);
+    createBranchWithRevision(BranchNameKey.create(project, "branch"), head);
+    PushOneCommit.Result baseCommit =
+        createChange(testRepo, "branch", "Add file", ADDED_FILE_NAME, ADDED_FILE_CONTENT, "");
+    baseCommit.assertOkStatus();
     RestResponse patchResp =
         userRestSession.get("/changes/" + baseCommit.getChangeId() + "/revisions/current/patch");
     patchResp.assertOK();
     String originalEncodedPatch = patchResp.getEntityContent();
     String originalDecodedPatch = new String(Base64.decode(patchResp.getEntityContent()), UTF_8);
     ApplyPatchPatchSetInput in = buildInput(originalEncodedPatch);
-    PushOneCommit.Result destChange = createChange();
 
     RestResponse resp =
         adminRestSession.post("/changes/" + destChange.getChangeId() + "/patch:apply", in);
 
     resp.assertOK();
     BinaryResult resultPatch = gApi.changes().id(destChange.getChangeId()).current().patch();
-    assertThat(removeHeader(resultPatch)).isEqualTo(removeHeader(originalDecodedPatch));
+    assertThat(cleanPatch(resultPatch)).isEqualTo(cleanPatch(originalDecodedPatch));
   }
 
   @Test
@@ -395,6 +401,45 @@
     GitPersonSubject.assertThat(person).email().isEqualTo(admin.email());
   }
 
+  @Test
+  public void applyPatchWithExplicitBase_OverrideParentId() throws Exception {
+    PushOneCommit.Result inputParent = createChange("Input parent", "file1", "content");
+    PushOneCommit.Result parent = createChange("Parent Change", "file2", "content");
+    parent.assertOkStatus();
+    PushOneCommit.Result dest = createChange("Destination Change", "file3", "content");
+    ApplyPatchPatchSetInput in = buildInput(ADDED_FILE_DIFF);
+    in.base = inputParent.getCommit().name();
+
+    gApi.changes().id(dest.getChangeId()).applyPatch(in);
+
+    ChangeInfo result = get(dest.getChangeId(), CURRENT_REVISION, CURRENT_COMMIT);
+    assertThat(result.revisions.get(result.currentRevision).commit.parents.get(0).commit)
+        .isEqualTo(inputParent.getCommit().name());
+
+    BinaryResult resultPatch = gApi.changes().id(dest.getChangeId()).current().patch();
+    assertThat(cleanPatch(resultPatch)).isEqualTo(ADDED_FILE_DIFF.trim());
+  }
+
+  @Test
+  public void applyPatchWithNoExplicitBase_overwritesLatestPatch() throws Exception {
+    PushOneCommit.Result dest = createChange("Destination Change", "ps1.txt", "ps1 content");
+    RevCommit originalParentCommit = dest.getCommit().getParent(0);
+    ApplyPatchPatchSetInput in = buildInput(ADDED_FILE_DIFF);
+
+    gApi.changes().id(dest.getChangeId()).applyPatch(in);
+
+    ChangeInfo result = get(dest.getChangeId(), CURRENT_REVISION, CURRENT_COMMIT, CURRENT_FILES);
+    assertThat(result.revisions.get(result.currentRevision).commit.parents.get(0).commit)
+        .isEqualTo(originalParentCommit.name());
+    assertThat(result.revisions.get(result.currentRevision).files.keySet())
+        .containsExactly(ADDED_FILE_NAME);
+    assertDiffForNewFile(
+        fetchDiffForFile(result, ADDED_FILE_NAME),
+        result.currentRevision,
+        ADDED_FILE_NAME,
+        ADDED_FILE_CONTENT);
+  }
+
   private void initDestBranch() throws Exception {
     String head = getHead(repo(), HEAD).name();
     createBranchWithRevision(BranchNameKey.create(project, ApplyPatchIT.DESTINATION_BRANCH), head);
@@ -425,11 +470,19 @@
     return gApi.changes().id(result.id).current().file(fileName).diff();
   }
 
-  private String removeHeader(BinaryResult bin) throws IOException {
-    return removeHeader(bin.asString());
+  private String cleanPatch(BinaryResult bin) throws IOException {
+    return cleanPatch(bin.asString());
   }
 
-  private String removeHeader(String s) {
-    return s.substring(s.lastIndexOf("\ndiff --git"), s.length() - 1);
+  private String cleanPatch(String s) {
+    return s
+        // Remove the header
+        .substring(s.lastIndexOf("\ndiff --git"), s.length() - 1)
+        // Remove "index NN..NN lines
+        .replaceAll("(?m)^index.*", "")
+        // Remove empty lines
+        .replaceAll("\n+", "\n")
+        // Trim
+        .trim();
   }
 }