U.S Ambassador James Warlick says he will step down on December 31 as co-chair of the OSCE Minsk Group mediating talks between Azerbaijan and Armenia over Nagorno-Karabakh.
"I will be a partner in Russia's largest and most prestigious law firm," Warlick said on Twitter on November 21.
Warlick has worked for the U.S. State Department for more than 30 years.
Egorov Puginsky Afanasiev & Partners said he will join the company's team in Washington in January.
"He will be responsible for working with U.S. and international clients, development of strategic client relationships, legislation, and public policy," a statement said.
Baku and Yerevan have been locked in a conflict over Azerbaijan's breakaway region of Nagorno-Karabakh for years.
Armenian-backed separatists seized the mainly Armenian-populated region from Azerbaijan during a war in the early 1990s that killed some 30,000 people.
Diplomatic efforts to settle the conflict have brought little progress.