Hamilton fought from 10th place on the grid to win ahead of championship rival Max Verstappen, but was placed under investigation for a seatbelt infringement after taking the chequered flag.
Hamilton stopped by a marshal to take a Brazilian flag which he held aloft from the cockpit and then took with him onto the podium to pay tribute to the local fans.
But the stewards placed Hamilton under investigation for breaching the International Sporting Code rule surrounding safety belts, leading to a post-race hearing.
It was announced that Hamilton had been fined €5,000 for the incident, with an additional €20,000 suspended through to the end of 2022.
“The driver of car 44, Lewis Hamilton, undid his seat belts on the in-lap at the end of the race,” the bulletin from the FIA stewards reads.
“While the Stewards are sympathetic to the desire to celebrate, it is fundamentally unsafe to undo the seatbelts while the car is in motion. Slow speeds in these cars are very fast for an unrestrained occupant.
“Further, Formula 1 drivers set the example for junior categories. It is critical that junior category drivers learn the importance of using all the safety devices of the car at all times.”
It brings to an end a turbulent weekend for Hamilton and Mercedes that saw the seven-time world champion get excluded from qualifying on Friday over a technical infringement.
Hamilton’s DRS failed the post-session FIA checks, with Mercedes suspecting this was due to a damaged part that it ordinarily would have been able to fix under parc ferme conditions had it been able to check the rear wing.
It left Hamilton 20th on the grid for the Saturday sprint race, but he recovered to finish fifth, meaning he started the Sunday grand prix from 10th place after an engine penalty was applied.
Hamilton then battled his way to victory after going wheel-to-wheel with title rival Verstappen, leading to the duo going off-track together on lap 48 as Verstappen tried to defend.
With victory in Brazil, Hamilton now trails Verstappen by just 14 points heading to the Qatar Grand Prix next weekend.