diff --git a/pr-checks/changelog.test.ts b/pr-checks/changelog.test.ts
index 45658154f..b88f9b0c0 100755
--- a/pr-checks/changelog.test.ts
+++ b/pr-checks/changelog.test.ts
@@ -9,13 +9,17 @@ import * as fs from "node:fs";
 import { describe, it } from "node:test";
 
 import {
+  addBodyLinesToUnreleasedSection,
+  ChangelogSection,
   EMPTY_CHANGELOG,
   getHeader,
   getReleaseDateString,
+  NO_CHANGES_STR,
   parseChangelog,
   processChangelogForBackports,
   renderChangelog,
   setVersionAndDate,
+  UNRELEASED_PLACEHOLDER,
 } from "./changelog";
 import { CHANGELOG_FILE } from "./config";
 
@@ -89,3 +93,78 @@ describe("processChangelogForBackports", async () => {
     assert.deepEqual(result.split("\n"), testChangelogResult.split("\n"));
   });
 });
+
+describe("addBodyLinesToUnreleasedSection", async () => {
+  function newChangelogWithSections(sections: ChangelogSection[]) {
+    return {
+      preamble: [],
+      sections,
+    };
+  }
+
+  await it("throws error if '[UNRELEASED]' section does not exist", async () => {
+    const emptyChangelog = newChangelogWithSections([]);
+    assert.throws(() => addBodyLinesToUnreleasedSection(emptyChangelog, []));
+
+    const releasedChangelog = newChangelogWithSections([
+      {
+        headerLine: "## Release 1.0.0",
+        bodyLines: [],
+      },
+      {
+        headerLine: "## Release 2.0.0",
+        bodyLines: [],
+      },
+      {
+        headerLine: "## Release 3.0.0",
+        bodyLines: [],
+      },
+    ]);
+    assert.throws(() => addBodyLinesToUnreleasedSection(releasedChangelog, []));
+  });
+
+  await it("overwrites 'No user facing changes.'", async () => {
+    const changelog = newChangelogWithSections([
+      {
+        headerLine: `## ${UNRELEASED_PLACEHOLDER}`,
+        bodyLines: ["", NO_CHANGES_STR, ""],
+      },
+    ]);
+
+    addBodyLinesToUnreleasedSection(changelog, ["- foo"]);
+
+    assert.equal(changelog.sections[0].bodyLines.length, 3);
+    assert.deepEqual(changelog.sections[0].bodyLines, ["", "- foo", ""]);
+  });
+
+  await it("does nothing if lines is empty", async () => {
+    const changelog = newChangelogWithSections([
+      {
+        headerLine: `## ${UNRELEASED_PLACEHOLDER}`,
+        bodyLines: ["", NO_CHANGES_STR, ""],
+      },
+    ]);
+    const changelogClone = structuredClone(changelog);
+
+    addBodyLinesToUnreleasedSection(changelog, []);
+
+    assert.deepEqual(changelog, changelogClone);
+  });
+
+  await it("inserts a line", async () => {
+    const changelog = newChangelogWithSections([
+      {
+        headerLine: `## ${UNRELEASED_PLACEHOLDER}`,
+        bodyLines: ["", "- Added a new dependency.", ""],
+      },
+    ]);
+    const lineToInsert = "- foo";
+
+    addBodyLinesToUnreleasedSection(changelog, [lineToInsert]);
+
+    assert.equal(changelog.sections[0].bodyLines.length, 4);
+    assert.ok(
+      changelog.sections[0].bodyLines.some((line) => line === lineToInsert),
+    );
+  });
+});
diff --git a/pr-checks/changelog.ts b/pr-checks/changelog.ts
index 826543024..ec765b4c8 100644
--- a/pr-checks/changelog.ts
+++ b/pr-checks/changelog.ts
@@ -132,6 +132,59 @@ export function parseChangelog(content: string): Changelog {
   return { preamble, sections };
 }
 
+/**
+ * Inserts the changenotes `notes` under the `[UNRELEASED]` section of `changelog`.
+ * If the section contains the stock message {@link NO_CHANGES_STR}, then
+ * `notes` will be inserted in place and the stock message will be deleted.
+ *
+ * This function will throw an exception if `[UNRELEASED]` does not exist.
+ *
+ * @param changelog The CHANGELOG object to modify.
+ * @param lines The changenotes to insert.
+ */
+export function addBodyLinesToUnreleasedSection(
+  changelog: Changelog,
+  lines: string[],
+) {
+  // Find the '[UNRELEASED]' section.
+  let unreleasedSection: ChangelogSection | undefined;
+  for (const section of changelog.sections) {
+    if (getHeader(section.headerLine) === UNRELEASED_PLACEHOLDER) {
+      unreleasedSection = section;
+      break;
+    }
+  }
+
+  // Ensure that the '[UNRELEASED]' section exists first.
+  if (unreleasedSection === undefined) {
+    throw Error(
+      "Cannot put changenotes into CHANGELOG.md's '[UNRELEASED]' section because it does not exist",
+    );
+  }
+
+  let insertAtIndex = 0;
+  let deleteCount = 0;
+
+  // If the section contains an empty line, preserve it -- insert afterward.
+  if (
+    unreleasedSection.bodyLines.length > 0 &&
+    unreleasedSection.bodyLines[0] === ""
+  ) {
+    insertAtIndex++;
+  }
+
+  // If the section contains the stock message 'No user facing changes.'
+  if (
+    lines.length > 0 &&
+    unreleasedSection.bodyLines.length > insertAtIndex &&
+    unreleasedSection.bodyLines[insertAtIndex].trim() === NO_CHANGES_STR
+  ) {
+    deleteCount++; // Delete the line by incrementing the delete marker.
+  }
+
+  unreleasedSection.bodyLines.splice(insertAtIndex, deleteCount, ...lines);
+}
+
 /**
  * Combines an array of lines into a single string by adding line breaks.
  */