The Gabonese government said in a Friday statement that Adamo, 62, died of a heart attack during a government meeting.
"We express our deepest condolences to the government of the Gabonese Republic, to the family and friends of Michael Adamo," the Russian embassy said in a statement on social media.
The embassy recalled that Adamo was a supporter of stronger ties with Russia.
Adamo had held several official positions in Gabon, including the post of defense minister and ambassador to the United States.