Colorado abolished its seldom-used death penalty on Monday, joining a growing number of states that have eschewed capital punishment as a deterrent to the most serious crimes.

Gov. Jared S. Polis, a Democrat, signed the repeal into law after it had reached his desk from the state legislature. It had passed the Senate in January and the House in February after several failed attempts to end capital punishment in the state.

Colorado had executed just one person since reinstating the death penalty in the mid-1970s: Gary Davis, who had been convicted of the rape and murder of Virginia May, was given a lethal injection in 1997.

On Monday, Mr. Polis also commuted the sentences of three men on death row — Robert Ray, Sir Mario Owens and Nathan Dunlap — to life in prison without the possibility of parole. He said in a statement that he wanted the law to be applied consistently.