The Hollywood star, who has been filming zombie thriller World War Z nearby, was inspired to give a little cheer for a friend's son, Zachary Gallagher, who spent the first nine weeks of his life in the medical centre.
"The fundraiser had finished, and I got a phone call from my best friend, who said, 'We have had a small donation.' I thought she was messing about and was going to say, 'My dad's given you a fiver' or something, then she said, 'It's from Brad Pitt!'," Zachary's mother, Karley Gallagher, told People.com.
"He said that he had seen the fundraiser mentioned in the local papers and seen my fliers and thought it was a really great cause and was just sorry he couldn't have been there. To have Brad assist us is more than we could ever have wished for," she added.
Event organiser Tricia Wilson-Hughes told the Sun that the Oscar-nominated actor also "signed postcards" and "promised to do a whip round of the film crew to send a donation from LA."
Brad, who was in New York for a screening of Killing Them Softly, revealed he spent Thanksgiving shooting on set.
"I didn't know it was Thanksgiving until the afternoon. It completely escaped me until someone brought me some English pumpkin pie," he told People.com.