Navalny, one of President Vladimir Putin's fiercest critics and a driving force behind last winter's mass protests, posted a photo of the listening device on his Twitter account on Monday.
Police officers were working at Navalny's offices to investigate how the device got planted there.
The 36-year-old anti-corruption activist was charged last month with theft for allegedly organizing a plan to steal about $500,000 in assets from a state timber company. He was ordered to remain in Moscow as authorities investigate him. Navalny has denied the charges.