U.S. Rep. Michael Grimm will resign from Congress, a decision that comes a week after he pleaded guilty to a tax-evasion charge.

“I do not believe that I can continue to be 100% effective in the next Congress, and therefore, out of respect for the office and the people I so proudly represent, it is time for me to start the next chapter of my life,” Mr. Grimm said in a statement late Monday night. The resignation will be effective Jan. 5, he said.

Mr. Grimm, a Republican, was re-elected to Staten Island’s 11th congressional district in November despite having been indicted in April. The charge stems from a restaurant he once owned on Manhattan’s Upper East Side.

Mr. Grimm, who once worked as an undercover agent with the Federal Bureau of Investigation on Wall Street, apologized last week but vowed to remain in Congress. “Let me be very clear—it’s wrong. I should not have done it, and I am truly sorry for it,” he said.

Former U.S. Rep. Michael McMahon, a Democrat, said Monday evening that he was considering running for Mr. Grimm’s seat. Mr. McMahon was elected to the seat in 2008 but lost it to Mr. Grimm in 2010.