President Trump signed a bill on Wednesday to require the detention of migrants who enter the country without authorization and are arrested or charged with certain crimes, the first legislation of his new presidency and an early victory in his effort to deliver a sweeping crackdown on immigration.

The new Republican-controlled Congress passed the legislation last week with some breakaway Democratic support, a sign of how the politics of immigration have moved to the right. Mr. Trump has long made curtailing immigration a centerpiece of his political identity, and the signing of the bill came after a flurry of executive orders that sought to systematically limit both legal and illegal entries into the United States.

“It’s a landmark law that we’re doing today,” Mr. Trump said in remarks at the White House before the signing. “It’s going to save countless innocent American lives.”

Mr. Trump, in a meandering monologue before the bill signing that ranged from boasting about his election results to bashing his predecessor, took time to address Ms. Riley’s family, which was present, as well as other “angel families,” whose family members were killed by people in the country illegally.

Supporters of President Trump holding signs with a photo of Laken Riley in Rome, Ga., in 2024. The law was named for Ms. Riley, a 22-year-old Georgia nursing student killed last year by a migrant from Venezuela.Credit…Nicole Craine for The New York Times

Mr. Trump said Ms. Riley was “respected by everybody” and her murderer was enabled by the “cruel policies of the previous administrations.”

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