MOSCOW — It was the most prominent political murder in Russia since President Vladimir V. Putin came to power.

Shortly before midnight on Feb. 27, 2015, the opposition leader Boris Y. Nemtsov was gunned down a few steps away from the Kremlin as he and his girlfriend were walking home across a bridge from Red Square.

On Thursday, a court in Moscow sentenced the Chechen former security services officer, who was convicted of pulling the trigger, to 20 years in prison; four accomplices were ordered to serve 11 to 19 years.

The shooting of Mr. Nemtsov, a charismatic opposition leader who antagonized the Kremlin, ended his two-decade democratic crusade that began in the wake of the Soviet Union’s collapse in 1991, and continued through the authoritarian rise of Mr. Putin.