Can I still make a 2024 HSA contribution now if my HDHP was only effective from Jan-Aug 2024? I understand that the limit will be prorated but I no longer have a HDHP.

No, it is too late to contribute to your Health Savings Account (HSA) account for 2024.   You have to be enrolled in High Deductible Health Plan in order to make contributions (HDHP) to an HSA account.   You can still own an HSA account; you just can't contribute without the HDHP.

There’s an annual limit to how much money you and your employer can add to your HSA.   Read this article to learn more about excess contributions.   HSA contributions made by you and your employer are in box 12 of your W-2 with the code W.  Don't enter the contributions on your W-2, again, when you go through the HSA interview.  

If you have an HSA, you may use these tax forms:

• Form 5498-SA: An informational form that you don’t need to enter
• Form 1099-SA: A form that reports how much HSA money you spent
• Form 8889: A form that TurboTax will generate for you if you contributed to, or spent money from, your HSA

What is a health savings account (HSA)? 

What is a high-deductible health plan (HDHP)?

The correct answer is Yes, you have until April 15, 2025 to make the prorated HSA contribution for 2024.  Being covered by an HDHP at the time of the deposit is not a requirement.