Julian Dunkerton donated the sum to the People's Vote group. His donation, the largest received by the organisation, will go towards funding opinion polls.
Writing in the Sunday Times, he said the public know Brexit will be a "disaster".
He said: "I'm putting some of my money behind the People's Vote campaign because I know we have a genuine chance to turn this around."
Mr Dunkerton added: "I will be paying for one of the most detailed public polling exercises ever undertaken by a campaign, so more of us have the confidence to demand the democratic right for our voices to be heard - to get a People's Vote on any Brexit deal."
He co-founded Superdry in 1985 from a market stall in Cheltenham.
The clothing retailer - famous for its hoodie tops and T-shirts - floated on the London Stock Exchange in 2010.
Mr Dunkerton said: "But if Brexit had happened 20 years earlier, Superdry would never have become the global success that it is."
