diff --git a/docs/changelog/120453.yaml b/docs/changelog/120453.yaml
new file mode 100644
index 0000000000000..48b34019f855d
--- /dev/null
+++ b/docs/changelog/120453.yaml
@@ -0,0 +1,5 @@
+pr: 120453
+summary: Support boolean strings for alias defination in CreateIndex API
+area: Indices APIs
+type: bug
+issues: []
diff --git a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.create/10_basic.yml b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.create/10_basic.yml
index 8645c91a51ad3..acf5641b4eecc 100644
--- a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.create/10_basic.yml
+++ b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.create/10_basic.yml
@@ -99,6 +99,8 @@
               is_write_index: false
             test_clias:
               is_write_index: true
+            test_dlias:
+              is_write_index: "true"
 
   - do:
       indices.get_alias:
@@ -107,6 +109,7 @@
   - is_false: test_index.aliases.test_alias.is_write_index
   - is_false: test_index.aliases.test_blias.is_write_index
   - is_true: test_index.aliases.test_clias.is_write_index
+  - is_true: test_index.aliases.test_dlias.is_write_index
 
 ---
 "Create index with invalid mappings":
diff --git a/server/src/main/java/org/elasticsearch/action/admin/indices/alias/Alias.java b/server/src/main/java/org/elasticsearch/action/admin/indices/alias/Alias.java
index a6959e5b59e06..b75aed3b08d7d 100644
--- a/server/src/main/java/org/elasticsearch/action/admin/indices/alias/Alias.java
+++ b/server/src/main/java/org/elasticsearch/action/admin/indices/alias/Alias.java
@@ -42,6 +42,14 @@ public class Alias implements Writeable, ToXContentFragment {
     private static final ParseField SEARCH_ROUTING = new ParseField("search_routing", "searchRouting", "search-routing");
     private static final ParseField IS_WRITE_INDEX = new ParseField("is_write_index");
     private static final ParseField IS_HIDDEN = new ParseField("is_hidden");
+    private static final Set<String> KNOWN_FIELDS = Set.of(
+        FILTER.getPreferredName(),
+        ROUTING.getPreferredName(),
+        INDEX_ROUTING.getPreferredName(),
+        SEARCH_ROUTING.getPreferredName(),
+        IS_WRITE_INDEX.getPreferredName(),
+        IS_HIDDEN.getPreferredName()
+    );
 
     private String name;
 
@@ -144,8 +152,12 @@ public Alias filter(QueryBuilder filterBuilder) {
      * Associates a routing value to the alias
      */
     public Alias routing(String routing) {
-        this.indexRouting = routing;
-        this.searchRouting = routing;
+        if (this.indexRouting == null) {
+            this.indexRouting = routing;
+        }
+        if (this.searchRouting == null) {
+            this.searchRouting = routing;
+        }
         return this;
     }
 
@@ -234,23 +246,14 @@ public static Alias fromXContent(XContentParser parser) throws IOException {
             if (token == XContentParser.Token.FIELD_NAME) {
                 currentFieldName = parser.currentName();
                 // check if there are any unknown fields
-                Set<String> knownFieldNames = Set.of(
-                    FILTER.getPreferredName(),
-                    ROUTING.getPreferredName(),
-                    INDEX_ROUTING.getPreferredName(),
-                    SEARCH_ROUTING.getPreferredName(),
-                    IS_WRITE_INDEX.getPreferredName(),
-                    IS_HIDDEN.getPreferredName()
-                );
-                if (knownFieldNames.contains(currentFieldName) == false) {
+                if (KNOWN_FIELDS.contains(currentFieldName) == false) {
                     throw new IllegalArgumentException("Unknown field [" + currentFieldName + "] in alias [" + alias.name + "]");
                 }
             } else if (token == XContentParser.Token.START_OBJECT) {
                 if (FILTER.match(currentFieldName, parser.getDeprecationHandler())) {
-                    Map<String, Object> filter = parser.mapOrdered();
-                    alias.filter(filter);
+                    alias.filter(parser.mapOrdered());
                 }
-            } else if (token == XContentParser.Token.VALUE_STRING) {
+            } else if (token.isValue()) {
                 if (ROUTING.match(currentFieldName, parser.getDeprecationHandler())) {
                     alias.routing(parser.text());
                 } else if (INDEX_ROUTING.match(currentFieldName, parser.getDeprecationHandler())) {
@@ -258,7 +261,6 @@ public static Alias fromXContent(XContentParser parser) throws IOException {
                 } else if (SEARCH_ROUTING.match(currentFieldName, parser.getDeprecationHandler())) {
                     alias.searchRouting(parser.text());
                 }
-            } else if (token == XContentParser.Token.VALUE_BOOLEAN) {
                 if (IS_WRITE_INDEX.match(currentFieldName, parser.getDeprecationHandler())) {
                     alias.writeIndex(parser.booleanValue());
                 } else if (IS_HIDDEN.match(currentFieldName, parser.getDeprecationHandler())) {
diff --git a/server/src/main/java/org/elasticsearch/rest/action/admin/indices/RestIndexPutAliasAction.java b/server/src/main/java/org/elasticsearch/rest/action/admin/indices/RestIndexPutAliasAction.java
index f68fbaee3e907..045a51965d350 100644
--- a/server/src/main/java/org/elasticsearch/rest/action/admin/indices/RestIndexPutAliasAction.java
+++ b/server/src/main/java/org/elasticsearch/rest/action/admin/indices/RestIndexPutAliasAction.java
@@ -95,6 +95,8 @@ public RestChannelConsumer prepareRequest(final RestRequest request, final NodeC
                         if ("filter".equals(currentFieldName)) {
                             filter = parser.mapOrdered();
                         }
+                    } else {
+                        throw new IllegalArgumentException("Unknown token [" + token + "]");
                     }
                 }
             }