diff --git a/server/src/main/java/org/opensearch/action/bulk/TransportBulkAction.java b/server/src/main/java/org/opensearch/action/bulk/TransportBulkAction.java
index 0068b9d3da172..d994a621fcff6 100644
--- a/server/src/main/java/org/opensearch/action/bulk/TransportBulkAction.java
+++ b/server/src/main/java/org/opensearch/action/bulk/TransportBulkAction.java
@@ -426,6 +426,12 @@ private boolean resolvePipelinesForActionRequests(
                 }
                 if (docRequest != null) {
                     if (updateRequest.docAsUpsert()) {
+                        // Save the original doc source before pipeline processing.
+                        // When the document already exists, the merge in UpdateHelper should use the
+                        // original user-provided source rather than the pipeline-modified source.
+                        // This fixes the inconsistency between single upsert and bulk upsert behavior
+                        // with ingest pipelines (see #10864).
+                        updateRequest.saveRawDocSource();
                         indexRequestHasPipeline |= ingestService.resolvePipelines(actionRequest, docRequest, metadata);
                     } else {
                         // In the case when doc as upsert is false or not defined, we only resolve system ingest pipelines.
diff --git a/server/src/main/java/org/opensearch/action/update/UpdateHelper.java b/server/src/main/java/org/opensearch/action/update/UpdateHelper.java
index 37d77649fadd6..7a0d723fd60f4 100644
--- a/server/src/main/java/org/opensearch/action/update/UpdateHelper.java
+++ b/server/src/main/java/org/opensearch/action/update/UpdateHelper.java
@@ -220,7 +220,23 @@ Result prepareUpdateIndexRequest(ShardId shardId, UpdateRequest request, GetResu
         final XContentType updateSourceContentType = sourceAndContent.v1();
         final Map<String, Object> updatedSourceAsMap = sourceAndContent.v2();
 
-        final boolean noop = !XContentHelper.update(updatedSourceAsMap, currentRequest.sourceAsMap(), detectNoop);
+        // When docAsUpsert is true and the document exists, use the original (pre-pipeline) doc source
+        // for merging. This ensures that the ingest pipeline does not corrupt the partial doc before
+        // it is merged with the existing document, matching the behavior of single upsert operations
+        // where the pipeline runs on the full merged document. See #10864.
+        final Map<String, Object> docSourceForMerge;
+        if (request.docAsUpsert() && request.rawDocSource() != null && request.rawDocSourceContentType() != null) {
+            Tuple<? extends MediaType, Map<String, Object>> rawSourceAndContent = XContentHelper.convertToMap(
+                request.rawDocSource(),
+                true,
+                request.rawDocSourceContentType()
+            );
+            docSourceForMerge = rawSourceAndContent.v2();
+        } else {
+            docSourceForMerge = currentRequest.sourceAsMap();
+        }
+
+        final boolean noop = !XContentHelper.update(updatedSourceAsMap, docSourceForMerge, detectNoop);
 
         // We can only actually turn the update into a noop if detectNoop is true to preserve backwards compatibility and to handle cases
         // where users repopulating multi-fields or adding synonyms, etc.
diff --git a/server/src/main/java/org/opensearch/action/update/UpdateRequest.java b/server/src/main/java/org/opensearch/action/update/UpdateRequest.java
index 9fe894a5c2db0..39d50d02becb6 100644
--- a/server/src/main/java/org/opensearch/action/update/UpdateRequest.java
+++ b/server/src/main/java/org/opensearch/action/update/UpdateRequest.java
@@ -48,6 +48,7 @@
 import org.opensearch.common.xcontent.XContentHelper;
 import org.opensearch.core.ParseField;
 import org.opensearch.core.common.Strings;
+import org.opensearch.core.common.bytes.BytesReference;
 import org.opensearch.core.common.io.stream.StreamInput;
 import org.opensearch.core.common.io.stream.StreamOutput;
 import org.opensearch.core.index.shard.ShardId;
@@ -158,6 +159,18 @@ public class UpdateRequest extends InstanceShardOperationRequest<UpdateRequest>
     @Nullable
     private IndexRequest doc;
 
+    /**
+     * Stores the original (pre-pipeline) source of the doc when docAsUpsert is true.
+     * This is used to ensure that when a bulk update with doc_as_upsert merges the doc
+     * into an existing document, the merge uses the original user-provided source rather
+     * than the pipeline-modified source.
+     */
+    @Nullable
+    private BytesReference rawDocSource;
+
+    @Nullable
+    private MediaType rawDocSourceContentType;
+
     public UpdateRequest() {}
 
     public UpdateRequest(StreamInput in) throws IOException {
@@ -191,6 +204,12 @@ public UpdateRequest(@Nullable ShardId shardId, StreamInput in) throws IOExcepti
         detectNoop = in.readBoolean();
         scriptedUpsert = in.readBoolean();
         requireAlias = in.readBoolean();
+        if (in.getVersion().onOrAfter(Version.V_3_6_0)) {
+            rawDocSource = in.readOptionalBytesReference();
+            if (rawDocSource != null) {
+                rawDocSourceContentType = in.readMediaType();
+            }
+        }
     }
 
     public UpdateRequest(String index, String id) {
@@ -839,6 +858,34 @@ public UpdateRequest docAsUpsert(boolean shouldUpsertDoc) {
         return this;
     }
 
+    /**
+     * Saves the original (pre-pipeline) source of the doc for use during update merging.
+     * When docAsUpsert is true and the document already exists, the merge should use the
+     * original source rather than the pipeline-modified source.
+     */
+    public void saveRawDocSource() {
+        if (this.doc != null) {
+            this.rawDocSource = this.doc.source();
+            this.rawDocSourceContentType = this.doc.getContentType();
+        }
+    }
+
+    /**
+     * Returns the original (pre-pipeline) source of the doc, or null if not saved.
+     */
+    @Nullable
+    public BytesReference rawDocSource() {
+        return this.rawDocSource;
+    }
+
+    /**
+     * Returns the content type of the original (pre-pipeline) doc source.
+     */
+    @Nullable
+    public MediaType rawDocSourceContentType() {
+        return this.rawDocSourceContentType;
+    }
+
     public boolean scriptedUpsert() {
         return this.scriptedUpsert;
     }
@@ -917,6 +964,12 @@ private void doWrite(StreamOutput out, boolean thin) throws IOException {
         out.writeBoolean(detectNoop);
         out.writeBoolean(scriptedUpsert);
         out.writeBoolean(requireAlias);
+        if (out.getVersion().onOrAfter(Version.V_3_6_0)) {
+            out.writeOptionalBytesReference(rawDocSource);
+            if (rawDocSource != null) {
+                rawDocSourceContentType.writeTo(out);
+            }
+        }
     }
 
     @Override