"Dadayev's guilt is confirmed by his confessions," the judge said.
Zaur Dadayev and Anzor Gubashev were detained early Saturday in connection with Nemtsov's assassination. Later in the day, reports emerged that two more alleged accomplices from southern Russia's Caucasus region have been apprehended.
Later the same day, a spokesman for the Security Council of Russia's Republic of Ingushetia told RIA Novosti that Dadayev served as deputy commander of a unit of the Chechen Republic Ministry of Internal Affairs's "Sever" battalion.
This decision can be appealed within three days. The decision regarding Dadayev's arrest was taken due to investigators' concerns that he may flee, threaten witnesses or destroy evidence.
Earlier Sunday, Russia's Investigative Committee was requesting permission to arrest five people involved in Nemtsov's murder.
Nemtsov, 55, who once held high positions in the Russian government in the late 1990s, was fatally shot in central Moscow late last Friday.
Shortly after the tragedy, Russian President Vladimir Putin ordered the Russian Investigative Committee, the Interior Ministry and the Federal Security Service to set up a joint team to investigate the murder. Investigators have been pursuing several motives behind Nemtsov's murder, including possible links to the Ukrainian crisis, contract killing or business-related disputes.