A 401(k) Roth (otherwise known as a "Designated Roth") is NOT an IRA, and does not get entered into the IRA contribution section at all. It is reported by your employer in box 12 (code AA) on your W-2 form that is entered into the W-2 section in TurboTax. The W-2 is the ONLY place that a 401(k) contribution is entered, unless you are self-employed with your own 401(k) plan, in which case it is entered on your 1040 line 28.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
