If a check for salaries paid is lost in the mail and re-issued the following year, which year does the employee pay taxes?

I think you made a mistake last year by not addressing this issue a year ago.

Payroll is tied to a specific tax year. When an employer does payroll, they send the employee a check and they also make a payment to the IRS in your name. The W-2 tells you how much taxable income was paid to you and how much was sent to the IRS on your behalf.

The company can’t, a year later, just change the W-2, as this has to match what was paid to the IRS.

I think the way you’ll need to fix this is to amend your 2023 tax return to include this income and tax withheld. If this results in an additional 2023 refund, then there won’t be any penalty or interest, either.

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All of the talk in the comments about whether or not the income belongs in 2023 or 2024 based on when he received the check are kind of irrelevant. As far as the individual goes, those rules are for non-employee income, such as business income. When you get a 1099-MISC or 1099-NEC that is not correct, it doesn’t matter a whole lot; your business reports the actual income you took “constructive receipt” of during the tax year. Not so with W-2 income: Your employee income on Form 1040 Line 1 has to match the sum of your W-2 forms; you can’t just add or remove income here. Likewise, your tax withheld reported later on the 1040 also has to match your W-2. If your W-2 isn’t right, you have to get it fixed.

From the company’s perspective, they sent the check and paid the OP in 2023. They don’t know if the OP lost the check or if the mail did, but they reported it and paid taxes on it in 2023 exactly like they should have. They have issued a replacement check, but that does not change any of their books. I doubt you’ll be able to get them to make any corrections to their 2023 and 2024 books at this time.

In general, you pay tax for income in the year you receive it. So you should include this income in your 2024 tax return.

Since this will not match your 2024 W-2, the IRS may send you a notice about this (I'm surprised they didn't notice it last year). You'll need to explain what happened, and they will most likely accept it. Mistakes like this probably happen all the time, they're surely use to it.

The two should cancel each other out, unless you were in different tax brackets in each year, so it's "no harm, no foul".