Footage obtained by The Sun newspaper appears to show the Mali midfielder inhaling the gas from a balloon at a party in November.
A Tottenham spokesperson told PA: "We are looking into the events and it will be dealt with as an internal matter."
Nitrous oxide was made a Class C drug in 2023 as part of the UK government's anti-social behaviour action plan, and possession of it for recreational use is an offence.
It is the second time Bissouma has been involved in a nitrous oxide controversy after also being caught in August 2024.
The 29-year-old apologised for a "severe lack of judgement" and was slapped with a one-game suspension.
Bissouma is currently sidelined following ankle surgery, but has been exiled from the Tottenham squad and has not yet made an appearance under Thomas Frank.
He was left out of Spurs' UEFA Super Cup squad in August for persistent lateness over pre-season.
Bissouma's last appearance was on May 25th against his former club Brighton, days after starting in the Europa League final win over Manchester United.