The court found that Zaur Dadayev shot Mr Nemtsov from a auto while the politician was walking with his girlfriend near the Kremlin, and that four other men acted as accomplices for supplying surveillance.
Prosecutor Maria Semenenko, who asked for Dadayev to receive a life sentence, said she may launch an appeal.
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Nemtsov was shot in the back four times with a pistol while walking across a bridge near the Kremlin with his girlfriend in February 2015.
Another suspect said to have taken part in the killing, Beslan Shavanov, died as police tried to arrest him. They were sentenced to 11, 14, 16, and 19 years in prison respectively.
He was Russia's deputy prime minister for four months in 1998, during the presidency of Boris Yeltsin.
Dadayev reportedly confessed early in the investigation to killing Nemtsov for insulting Islam and Chechnya's leader, Ramzan Kadyrov, but later recanted his confession.
Despite the guilty verdict handed out by the court last week, Nemtsov's relatives and supporters argue the men in the court-room are mere pawns.
"The shortcoming of this sentencing is that those who ordered and organised this crime are not in the dock", said Vadim Prokhorov, a lawyer for Nemtsov's daughter Zhanna.
As he quickly rose through the ranks of the Russian government in the 1990s after the fall of the Soviet Union, Mr. Nemtsov turned into an advocate of democratic reforms. The family of Nemtsov has repeatedly blamed Chechnya's leader Ramzan Kadyrov for the assassination.