Beginning January 1, 2026, certain higher‑earning employees who make catch‑up contributions to employer‑sponsored retirement plans (e.g., 401(k), 403(b), governmental 457(b)) must make those catch‑up contributions on a Roth (after‑tax) basis. This requirement comes from SECURE 2.0 Act §603 and is now fully clarified through final IRS regulations issued in September 2025.