The Hollywood actor strolled into a charity shop and made a £1,000 donation during a break from filming.
Staff at the Cancer Research shop in Sunningdale, Ascot were stunned after the Aussie hunk handed over the cheque.
Russell dropped by the store which is next to a café he's frequenting while filming nearby.
Manager Julia Deane told the BBC he chatted quietly with staff before joining a queue of customers.
"The lady at the till didn't recognise him. He asked if we took donations and she told him that we do in the back of the shop.
"She gave him the book to fill in his name and he wrote Russell Crowe."
Once known for his volatile nature, the leading man now often hits the headlines for his big-hearted gestures.
Working in Wales this summer, he left a generous tip running into the hundreds for pub staff.
Later he also gave movie worker Denise Yarde £5,000 so she could buy a new car. She told him off for making fun of her when her old one developed a fault and went up in flames.
When she pointed out he'd never have to worry about financing a new set of wheels, he offered her the money.
In 2001, the father of two donated $200,000 so a struggling primary school near his home in rural Australia could get a swimming pool.
His sympathies were sparked when a young pupil drowned and, believing the pool would help students become better swimmers, he stumped up the cash.