The legislation replaces lethal injection with a sentence of life in prison without the possibility of parole.
“Faced with the reality that our system for imposing the death penalty can never be perfect, my conscience compels me to replace the death penalty with a solution that keeps society safe,” Mr. Richardson said at a news conference in the Capitol.
The governor, a Democrat, faced a deadline of midnight for making a decision on the bill that lawmakers sent him last week.
New Mexico is only the second state to ban executions since the United States Supreme Court reinstated the death penalty in 1976. New Jersey was the first, in 2007. In all, 15 states now bar capital punishment.
New Mexico has executed only one person since 1960, Terry Clark, a child killer, in 2001.
Two men are currently on death row, Robert Fry of Farmington and Timothy Allen of Bloomfield. Their sentences are not affected by the new law
Mr. Richardson, who formerly supported capital punishment, said his decision was “extremely difficult,” and he solicited advice over the weekend from state residents.
Among those urging the governor to sign the bill was the U.S. Conference of Catholic Bishops. Officials of the Roman Catholic Church lobbied hard for repeal.
Lt. Gov. Diane D. Denish, a Democrat, said she delivered a handwritten note to the governor on Wednesday indicating her support for repeal.
The New Mexico Sheriffs’ and Police Association opposed repeal, saying capital punishment deterred violence against police officers, jailers and prison guards. District attorneys also opposed the legislation, arguing that the death penalty was a useful prosecutorial tool.
New Mexico was one of several states considering repealing the death penalty this year. In Kansas, a bill to do so failed to clear the Senate this week.
