diff --git a/src/main/java/org/openrewrite/jenkins/AddJellyXmlDeclaration.java b/src/main/java/org/openrewrite/jenkins/AddJellyXmlDeclaration.java
new file mode 100644
index 0000000..700dddf
--- /dev/null
+++ b/src/main/java/org/openrewrite/jenkins/AddJellyXmlDeclaration.java
@@ -0,0 +1,84 @@
+/*
+ * Copyright 2024 the original author or authors.
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * https://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.openrewrite.jenkins;
+
+import org.openrewrite.ExecutionContext;
+import org.openrewrite.Recipe;
+import org.openrewrite.text.PlainText;
+import org.openrewrite.text.PlainTextVisitor;
+
+/**
+ * Recipe to add an XML declaration to Jelly files.
+ */
+public class AddJellyXmlDeclaration extends Recipe {
+    private static final String JELLY_DECLARATION = "<?jelly escape-by-default='true'?>";
+
+    /**
+     * Returns the display name of the recipe.
+     *
+     * @return the display name of the recipe
+     */
+    @Override
+    public String getDisplayName() {
+        return "Add XML declaration to Jelly files";
+    }
+
+    /**
+     * Returns the description of the recipe.
+     *
+     * @return the description of the recipe
+     */
+    @Override
+    public String getDescription() {
+        return "Ensure the XML declaration `<?jelly escape-by-default='true'?>` is present in all `.jelly` files.";
+    }
+
+    /**
+     * Returns a visitor that adds the XML declaration to Jelly files.
+     *
+     * @return a PlainTextVisitor that adds the XML declaration
+     */
+    @Override
+    public PlainTextVisitor<ExecutionContext> getVisitor() {
+        return new PlainTextVisitor<ExecutionContext>() {
+
+            /**
+             * Visits the text and adds the XML declaration if necessary.
+             *
+             * @param text the PlainText object representing the file content
+             * @param ctx the execution context
+             */
+            @Override
+            public PlainText visitText(PlainText text, ExecutionContext ctx) {
+                if (text.getSourcePath().toString().endsWith(".jelly")) {
+                    String content = text.getText().trim();
+                    if (content.isEmpty()) {
+                        return text.withText(JELLY_DECLARATION);
+                    }
+                    String lineEnding = content.contains("\r\n") ? "\r\n" : "\n";
+                    if (content.toLowerCase().matches("^<\\?jelly\\s+[^>]*>") && !content.startsWith(JELLY_DECLARATION)) {
+                        content = content.substring(content.indexOf(lineEnding) + lineEnding.length());
+                    }
+                    if (!content.startsWith(JELLY_DECLARATION)) {
+                        content = JELLY_DECLARATION + lineEnding + content;
+                        return text.withText(content);
+                    }
+                }
+                return text;
+            }
+        };
+    }
+}
diff --git a/src/test/java/org/openrewrite/jenkins/AddJellyXmlDeclarationTest.java b/src/test/java/org/openrewrite/jenkins/AddJellyXmlDeclarationTest.java
new file mode 100644
index 0000000..beadaf3
--- /dev/null
+++ b/src/test/java/org/openrewrite/jenkins/AddJellyXmlDeclarationTest.java
@@ -0,0 +1,95 @@
+/*
+ * Copyright 2024 the original author or authors.
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * https://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.openrewrite.jenkins;
+
+import org.junit.jupiter.api.Test;
+import org.openrewrite.DocumentExample;
+import org.openrewrite.test.RecipeSpec;
+import org.openrewrite.test.RewriteTest;
+
+import static org.openrewrite.test.SourceSpecs.text;
+
+class AddJellyXmlDeclarationTest implements RewriteTest {
+
+    @Override
+    public void defaults(RecipeSpec spec) {
+        spec.recipe(new AddJellyXmlDeclaration());
+    }
+
+    @DocumentExample
+    @Test
+    void addJellyXmlDeclaration() {
+        rewriteRun(
+          //language=xml
+          text(
+            """
+              <j:jelly xmlns:j="jelly:core">
+                <h1>Simple Example</h1>
+              </j:jelly>
+              """,
+            """
+              <?jelly escape-by-default='true'?>
+              <j:jelly xmlns:j="jelly:core">
+                <h1>Simple Example</h1>
+              </j:jelly>
+              """,
+            spec -> spec.path("example.jelly")
+          )
+        );
+    }
+
+    @Test
+    void addXmlDeclarationToJellyFile() {
+        rewriteRun(
+          spec -> spec.expectedCyclesThatMakeChanges(1),
+          //language=xml
+          text(
+            """
+              <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define">
+                  <st:contentType value="text/html"/>
+                  <h1>Hello, World!</h1>
+              </j:jelly>
+              """,
+            """
+              <?jelly escape-by-default='true'?>
+              <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define">
+                  <st:contentType value="text/html"/>
+                  <h1>Hello, World!</h1>
+              </j:jelly>
+              """,
+            spec -> spec.path("example.jelly")
+          )
+        );
+    }
+
+    @Test
+    void doNotAddXmlDeclarationIfAlreadyPresent() {
+        //language=xml
+        rewriteRun(
+          spec -> spec.expectedCyclesThatMakeChanges(0),
+          text(
+            """
+              <?jelly escape-by-default='true'?>
+              <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define">
+                  <st:contentType value="text/html"/>
+                  <h1>Hello, World!</h1>
+              </j:jelly>
+              """,
+            spec -> spec.path("example.jelly")
+          )
+        );
+    }
+}