Tricia Wilson-Hughes, who works at a gift shop in Lulworth, is raising money for Southampton hospital's neonatal unit where a friend's baby was treated, reported BBC online.
She had managed to raise 40 pounds from the crew of the zombie film 'World War Z', during a whip round.
But then she saw Pitt's security guard and asked him if the star could donate and the guard returned with 700 pounds.
Wilson-Hughes said she was worried the filming might impact on a fundraising event in Lulworth on Saturday and wanted to see if the actor could help.
"I wasn't too bothered if I didn't see Brad Pitt... [but] I felt I had to do something for the charity. I was going round the crew as they passed the gift shop. They were putting in 5 pound notes...
One asked me if we took cards."
She said when she spotted Pitt's security guards she asked for a small donation "of maybe 50 pounds".
"[The guard] came back about 10 minutes later and said this is off Brad for your charity. He said there's 700 pounds in there, I couldn't believe it, I was completely dumbstruck. He said if he had more he would have given it."