Tax season just ended, but another deadline will soon arrive for tens of thousands of California residents: those who never filed their 2020 tax returns but who may be due a refund. 

This week the Internal Revenue Service issued a “final reminder” of the May 17 deadline for U.S. taxpayers to file for more than $1 billion in unclaimed refunds from the 2020 tax year nationwide — nearly $100 million of that in California.

And some people may be eligible for unclaimed tax credits on top of their refunds, according to the IRS.

Some taxpayers may have forgotten to file because of “extremely unusual situations during the pandemic,” which began during the 2020 tax year, IRS Commissioner Danny Werfel said in a news release. They might include students and part-time workers who “may not realize they may be owed a refund,” he said. 

However, Werfel stressed that time is running out: Taxpayers must file their 2020 tax returns by May 17 to receive a refund check. They will not be penalized for filing late if a refund is due, the IRS said.

Normally, taxpayers have three years from the original April deadline to file and claim their tax refunds. If they miss that window, the U.S. Treasury claims the money. 

But this year, taxpayers are getting a little more time. That’s because the IRS extended the original 2020 tax filing deadline of April 15, 2021, by a month due to the COVID-19 pandemic — which also pushed back the three-year window deadline for unfiled returns, officials said.

Of all 50 states and the District of Columbia, California has the second-highest estimated number of people due a potential refund as well as the second-highest total amount — about 88,200 people and $94 million, according to IRS data. Texas is No. 1 with about 93,400 people and $107 million in potential refunds, excluding credits, the data shows.

Filers could also be eligible to receive more money on top of their refund check, including the earned income tax credit and COVID-era credits, among others, the IRS said.

People who did not receive one or more of the three stimulus payments in 2020 and 2021, or received less than what they were eligible for, can check to see whether they are eligible for the Recovery Rebate Credit.

Low- to moderate-income earners may also be qualified to claim the earned income tax credit.

For more information about unfiled tax returns, visit the IRS website to receive free help from the agency’s Interactive Tax Assistance webpage or its FAQ webpage.

Reach Jessica Flores: jessica.flores@sfchronicle.com; Twitter: @jesssmflores