The 32-year-old, crowned world champion in Mexico last week for a fourth time, says that the incident took place on Friday evening when his team were leaving the Sao Paulo circuit and heading back to their nearby hotel.
Nobody was hurt in the incident, but some valuables are said to have been taken from the team bus ahead of Sunday's race.
Posting on Twitter, Hamilton said: "Some of my team were held up at gun point last night leaving the circuit here in Brazil. Gun shots fired, gun held at ones head. This is so upsetting to hear.
"Please say a prayer for my guys who are here as professionals today even if shaken. This happens every single year here. F1 and the teams need to do more, there's no excuse!"
Mercedes later released a statement, which read: "One minibus was robbed at gunpoint. Valuables were stolen but everybody is safe and uninjured which is the main thing."
Hamilton's fellow Brit Jenson Button escaped an armed attack on his car in the same city in 2010, while ex-F1 tycoon Bernie Ecclestone's mother-in-law was taken hostage for a reported £28m ransom last year.