I had a high deductible health plan in 2025, but do not in 2026. Can I still make an HSA contribution after Jan 1, 2026 but for the 2025 tax year?

[dmertz]

Yes, up to the pro-rated amount for the number of months that you were HSA eligible in 2025.

[AnnetteB6]

Yes, you are still eligible to make a contribution for the period of time that you were covered by a High Deductible Health Plan (HDHP) during 2025.  

Take a look at the following information from IRS Publication 969 for reference.  It is written for tax year 2024, but provides confirmation that you can make a contribution for a period of time which you were eligible even if you are not eligible at the time the contribution is made for the previous tax year:  

Publication 969 - When to Contribute

@erictvedt