diff --git a/.github/workflows/release-please.yml b/.github/workflows/release-please.yml
index 475d77aa..6f472e15 100644
--- a/.github/workflows/release-please.yml
+++ b/.github/workflows/release-please.yml
@@ -18,6 +18,35 @@ jobs:
         with:
           release-type: python
 
+      - name: Checkout release PR
+        if: ${{ steps.release.outputs.prs_created == 'true' }}
+        uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      - name: Normalize release PR pyproject prerelease version
+        if: ${{ steps.release.outputs.prs_created == 'true' }}
+        env:
+          RELEASE_PR_BRANCH: release-please--branches--${{ github.ref_name }}
+        run: |
+          set -euo pipefail
+
+          git fetch origin "${RELEASE_PR_BRANCH}:${RELEASE_PR_BRANCH}"
+          git switch "${RELEASE_PR_BRANCH}"
+
+          python3 scripts/normalize_pyproject_prerelease.py pyproject.toml
+
+          if git diff --quiet -- pyproject.toml; then
+            echo "pyproject.toml already uses a PEP 440-compatible version"
+            exit 0
+          fi
+
+          git config user.name "github-actions[bot]"
+          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
+          git add pyproject.toml
+          git commit -m "chore: normalize prerelease version to PEP 440"
+          git push origin "HEAD:${RELEASE_PR_BRANCH}"
+
       # Publish to PyPI when a release is created
       - name: Checkout
         if: ${{ steps.release.outputs.release_created }}
diff --git a/release-please-config.json b/release-please-config.json
index 10a09797..89e087ad 100644
--- a/release-please-config.json
+++ b/release-please-config.json
@@ -6,7 +6,7 @@
       "changelog-path": "CHANGELOG.md",
       "bump-minor-pre-major": true,
       "bump-patch-for-minor-pre-major": false,
-      "versioning": "default",
+      "versioning": "prerelease",
       "prerelease": true,
       "prerelease-type": "beta"
     }
diff --git a/scripts/normalize_pyproject_prerelease.py b/scripts/normalize_pyproject_prerelease.py
new file mode 100644
index 00000000..2cf3599b
--- /dev/null
+++ b/scripts/normalize_pyproject_prerelease.py
@@ -0,0 +1,102 @@
+#!/usr/bin/env python3
+"""Normalize release-please prerelease versions in pyproject.toml."""
+
+from __future__ import annotations
+
+import argparse
+import re
+from pathlib import Path
+
+_SEMVER_PRERELEASE_RE = re.compile(
+    r"^(?P<release>\d+\.\d+\.\d+)-(?P<label>alpha|beta|rc)\.(?P<number>0|[1-9]\d*)$"
+)
+_PYPROJECT_SECTION_RE = re.compile(r"^\s*\[(?P<section>[^\]]+)\]\s*$")
+_PYPROJECT_VERSION_LINE_RE = re.compile(
+    r'^(?P<prefix>\s*version\s*=\s*")'
+    r"(?P<version>\d+\.\d+\.\d+-(?:alpha|beta|rc)\.(?:0|[1-9]\d*))"
+    r'(?P<suffix>"\s*)$'
+)
+_PEP440_LABELS = {
+    "alpha": "a",
+    "beta": "b",
+    "rc": "rc",
+}
+
+
+def pep440_prerelease(version: str) -> str:
+    """Convert a SemVer prerelease version to PEP 440 when needed."""
+    match = _SEMVER_PRERELEASE_RE.fullmatch(version)
+    if not match:
+        return version
+
+    label = _PEP440_LABELS[match.group("label")]
+    return f"{match.group('release')}{label}{match.group('number')}"
+
+
+def normalize_pyproject_text(text: str) -> str:
+    """Normalize a pyproject.toml project version line if it is a prerelease."""
+
+    lines = text.splitlines(keepends=True)
+    current_section: str | None = None
+    for index, line in enumerate(lines):
+        content = line.removesuffix("\n")
+        newline = "\n" if line.endswith("\n") else ""
+        section_match = _PYPROJECT_SECTION_RE.match(content)
+        if section_match:
+            current_section = section_match.group("section").strip()
+            continue
+
+        if current_section != "project":
+            continue
+
+        version_match = _PYPROJECT_VERSION_LINE_RE.match(content)
+        if not version_match:
+            continue
+
+        lines[index] = (
+            f"{version_match.group('prefix')}"
+            f"{pep440_prerelease(version_match.group('version'))}"
+            f"{version_match.group('suffix')}"
+            f"{newline}"
+        )
+
+    return "".join(lines)
+
+
+def normalize_pyproject(path: Path) -> bool:
+    """Normalize pyproject.toml in place.
+
+    Returns True when the file changed.
+    """
+    text = path.read_text(encoding="utf-8")
+    normalized = normalize_pyproject_text(text)
+    if normalized == text:
+        return False
+
+    path.write_text(normalized, encoding="utf-8")
+    return True
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(
+        description="Normalize release-please SemVer prereleases to PEP 440 in pyproject.toml."
+    )
+    parser.add_argument(
+        "path",
+        nargs="?",
+        default="pyproject.toml",
+        type=Path,
+        help="Path to pyproject.toml.",
+    )
+    args = parser.parse_args()
+
+    changed = normalize_pyproject(args.path)
+    if changed:
+        print(f"Normalized prerelease version in {args.path}")
+    else:
+        print(f"No prerelease normalization needed for {args.path}")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/tests/test_normalize_pyproject_prerelease.py b/tests/test_normalize_pyproject_prerelease.py
new file mode 100644
index 00000000..afd86bb0
--- /dev/null
+++ b/tests/test_normalize_pyproject_prerelease.py
@@ -0,0 +1,51 @@
+from pathlib import Path
+
+from scripts.normalize_pyproject_prerelease import (
+    normalize_pyproject,
+    normalize_pyproject_text,
+    pep440_prerelease,
+)
+
+
+def test_pep440_prerelease_converts_supported_semver_labels() -> None:
+    assert pep440_prerelease("7.0.0-alpha.1") == "7.0.0a1"
+    assert pep440_prerelease("7.0.0-beta.2") == "7.0.0b2"
+    assert pep440_prerelease("7.0.0-rc.3") == "7.0.0rc3"
+
+
+def test_pep440_prerelease_leaves_other_versions_unchanged() -> None:
+    assert pep440_prerelease("7.0.0") == "7.0.0"
+    assert pep440_prerelease("7.0.0b1") == "7.0.0b1"
+    assert pep440_prerelease("7.0.0-dev.1") == "7.0.0-dev.1"
+
+
+def test_normalize_pyproject_text_changes_only_project_version_line() -> None:
+    text = """\
+[project]
+name = "adcp"
+version = "7.0.0-beta.1"
+
+[tool.example]
+version = "1.0.0-beta.1"
+"""
+
+    assert (
+        normalize_pyproject_text(text)
+        == """\
+[project]
+name = "adcp"
+version = "7.0.0b1"
+
+[tool.example]
+version = "1.0.0-beta.1"
+"""
+    )
+
+
+def test_normalize_pyproject_reports_whether_file_changed(tmp_path: Path) -> None:
+    pyproject = tmp_path / "pyproject.toml"
+    pyproject.write_text('[project]\nversion = "6.1.0-beta.1"\n', encoding="utf-8")
+
+    assert normalize_pyproject(pyproject) is True
+    assert pyproject.read_text(encoding="utf-8") == '[project]\nversion = "6.1.0b1"\n'
+    assert normalize_pyproject(pyproject) is False