Air steward Michael Nicholas, 45, said he was walking back to a hotel from a barbecue with 10 other colleagues when the gunman pounced.
The attacker held the gun against one of the crew’s face before forcing him and his son to hand over their phones and money, the Sun reported.
Describing the ordeal on his Facebook page, Mr Nicholas wrote: "One of our crew had a gun held to his face while his son and him were robbed.
“Half of us were unaware anything was going on behind us until we heard the commotion. Luckily, the son gave the robber his phone and money and they left.
“We are all safe and well but a little shaken. The local police refused to attend the hotel and BA security are on the case and dealing with this.”
Mr Nicholas, from Preston Candover, Hampshire, urged fellow crew members to take care if they ventured out of the hotel in Rio after dark and travel by taxi.
The Foreign Office website advises travellers to Brazil to hand over any valuables if threatened and not to resist attackers as they may be armed or under the influence of drugs.
A BA spokesman told the Sun: “We are supporting our crew and are thankful that they weren’t harmed.
“We take the safety of our pilots and cabin crew very seriously and issue them with information and advice for all the destinations to which we fly – specifying any extra precautions that should be taken in particular cities.
"We will be reiterating our advice to crew operating to Rio de Janeiro, including that they should remain vigilant when outside the hotel. We will now also be advising that they book taxis when travelling around the city.”
