diff --git a/CHANGELOG.md b/CHANGELOG.md
index cbf0130517428..03b4b87305280 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -62,6 +62,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix snapshot _status API to return correct status for partial snapshots ([#12812](https://github.com/opensearch-project/OpenSearch/pull/12812))
 - Improve the error messages for _stats with closed indices ([#13012](https://github.com/opensearch-project/OpenSearch/pull/13012))
 - Ignore BaseRestHandler unconsumed content check as it's always consumed. ([#13290](https://github.com/opensearch-project/OpenSearch/pull/13290))
+- Fix mapper_parsing_exception when using flat_object fields with names longer than 11 characters ([#13259](https://github.com/opensearch-project/OpenSearch/pull/13259))
 
 ### Security
 
diff --git a/server/src/main/java/org/opensearch/common/xcontent/JsonToStringXContentParser.java b/server/src/main/java/org/opensearch/common/xcontent/JsonToStringXContentParser.java
index 9b2bd06a88e2e..998122d9e5c43 100644
--- a/server/src/main/java/org/opensearch/common/xcontent/JsonToStringXContentParser.java
+++ b/server/src/main/java/org/opensearch/common/xcontent/JsonToStringXContentParser.java
@@ -106,8 +106,9 @@ private void parseToken(StringBuilder path, String currentFieldName) throws IOEx
                 // skip
             } else if (this.parser.currentToken() == Token.START_OBJECT) {
                 parseToken(path, currentFieldName);
-                int dotIndex = path.lastIndexOf(DOT_SYMBOL);
-                if (dotIndex != -1) {
+                int dotIndex = path.lastIndexOf(DOT_SYMBOL, path.length());
+
+                if (dotIndex != -1 && path.length() > currentFieldName.length()) {
                     path.setLength(path.length() - currentFieldName.length() - 1);
                 }
             } else {
@@ -117,8 +118,8 @@ private void parseToken(StringBuilder path, String currentFieldName) throws IOEx
                 parseValue(parsedFields);
                 this.valueList.add(parsedFields.toString());
                 this.valueAndPathList.add(path + EQUAL_SYMBOL + parsedFields);
-                int dotIndex = path.lastIndexOf(DOT_SYMBOL);
-                if (dotIndex != -1) {
+                int dotIndex = path.lastIndexOf(DOT_SYMBOL, path.length());
+                if (dotIndex != -1 && path.length() > currentFieldName.length()) {
                     path.setLength(path.length() - currentFieldName.length() - 1);
                 }
             }
diff --git a/server/src/test/java/org/opensearch/index/mapper/FlatObjectFieldDataTests.java b/server/src/test/java/org/opensearch/index/mapper/FlatObjectFieldDataTests.java
index e318ca5e953a3..03ce1eb7252de 100644
--- a/server/src/test/java/org/opensearch/index/mapper/FlatObjectFieldDataTests.java
+++ b/server/src/test/java/org/opensearch/index/mapper/FlatObjectFieldDataTests.java
@@ -54,6 +54,49 @@ public void testDocValue() throws Exception {
         assertEquals(1, valueReaders.size());
     }
 
+    public void testLongFieldNameWithHashArray() throws Exception {
+        String mapping = XContentFactory.jsonBuilder()
+            .startObject()
+            .startObject("test")
+            .startObject("properties")
+            .startObject("field")
+            .field("type", FIELD_TYPE)
+            .endObject()
+            .endObject()
+            .endObject()
+            .endObject()
+            .toString();
+        final DocumentMapper mapper = mapperService.documentMapperParser().parse("test", new CompressedXContent(mapping));
+
+        XContentBuilder json = XContentFactory.jsonBuilder()
+            .startObject()
+            .startObject("field")
+            .startObject("detail")
+            .startArray("fooooooooooo")
+            .startObject()
+            .field("name", "baz")
+            .endObject()
+            .startObject()
+            .field("name", "baz")
+            .endObject()
+            .endArray()
+            .endObject()
+            .endObject()
+            .endObject();
+
+        ParsedDocument d = mapper.parse(new SourceToParse("test", "1", BytesReference.bytes(json), MediaTypeRegistry.JSON));
+        writer.addDocument(d.rootDoc());
+        writer.commit();
+
+        IndexFieldData<?> fieldData = getForField("field");
+        List<LeafReaderContext> readers = refreshReader();
+        assertEquals(1, readers.size());
+
+        IndexFieldData<?> valueFieldData = getForField("field._value");
+        List<LeafReaderContext> valueReaders = refreshReader();
+        assertEquals(1, valueReaders.size());
+    }
+
     @Override
     protected String getFieldDataType() {
         return FIELD_TYPE;