The second stimulus is the $600 per person enacted by Congress on 12/27/2020 and that most people received in January 2021.
Both stimulus payments are advanced payments of the Recovery Rebate Credit. And you have report the second stimulus on your 2020 tax return, even if you received it in 2021.
Please read this IRS document for more information.
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Congress passed the 2nd stimulus bill at the end of 2020. The 2nd stimulus checks started going out at the very end of December 2020 and continued into early January 2021. When you fill out the recovery rebate credit it is important for you to enter the entire total of the amount you have received for yourself, your spouse, and your dependent children so far. Entering the wrong amount might delay your 2020 refund.