Andrey Karlov was speaking at a photo exhibition event in the capital when he was shot by a Turkish off-duty policeman.
The US embassy in Ankara and other consulates in Turkey were closed on Tuesday after an individual fired shots in front of the mission in the capital.
Turkish police have detained the man, who took out a shotgun he hid in his coat and fired about eight rounds into the air before the embassy’s security guards intervened and apparently overpowered him.
No one was hurt in the incident, which occurred at 3:50am local time (00:50 GMT), hours after a Turkish policeman who condemned Russia’s military role in Syria fatally shot Russian Ambassador Andrey Karlov at a photo exhibition in Ankara.
The US embassy is just across the street from the art exhibition building where the attack on the ambassador occurred. It was not immediately clear if the two incidents were connected.
The leaders of Turkey and Russia have described the attack as an attempt to disrupt efforts to repair ties between their countries, which have backed opposing sides in the Syrian civil war.
The American embassy said its missions in Ankara, Istanbul, and the southern city of Adana would be “closed for normal operations on Tuesday”.