Former deputy prime minister Mr Nemtsov, a fierce critic of President Vladimir Putin, was shot dead just metres from the Kremlin as he walked home at night with his girlfriend in February 2015.
The murder in Moscow was the most high-profile political killing in Russia since Mr Putin rose to power 17 years ago.
The jury of 12 ruled Zaur Dadayev, Shadid and Anzor Gubashev, Temirlan Eskerkhanov and Khamzat Bakhayev, all ethnic Chechens from Russia’s volatile North Caucasus, carried out the hit as part of an organised gang.
Dadayev, a former officer in an interior ministry battalion in Chechnya, was found guilty of firing the four fatal shots.
The jury’s decision was reached by majority vote after they first failed to come to unanimous decisions on the long list of charges against the defendants at the end of 10 months of hearings.
Mr Nemtsov, 55, who was murdered as he walked across a bridge, had been working on a report on Russia’s role in the Ukraine.
The ruling gives some answers to questions about the murder but Mr Nemtsov’s supporters insist that despite the trial, those who ordered the charismatic politician’s death have yet to be brought to justice.
Lawyers for his family said they were convinced that all of the accused, with the exception of Bakhayev, were involved.
But they still insist the evidence clearly shows that those close to Kremlin-loyal Chechen strongman Ramzan Kadyrov – or Kadyrov himself – were actually behind the assassination.
Mr Nemtsov’s supporters welcomed the verdict, but said Dadayev and the others were low-level operatives and the case remained unsolved because those who had ordered, financed and organised the hit had not been caught.
After the verdict, Vadim Prokhorov, a lawyer of the late politician’s daughter, said: “It’s the biggest crime of the century and yet they haven’t identified the real organisers or those who ordered it.”