They join Roxy Music singer Bryan Ferry who became a trustee of Glasgow School of Art more than a year ago.
The appeal cash will be used to help rebuild the Mackintosh Building which was devastated by fire.
Fire fighters managed to save 90% of the world famous building and 70% of the contents but it is estimated it could cost up to £35m to restore.
Art school director Professor Tom Inns said: "We are delighted both Peter Capaldi, one of our most celebrated alumni and Brad Pitt, whose admiration for Mackintosh is well known, have both joined our campaign and will be trustees on the Mackintosh appeal.
"Our priority to date has been to address the immediate physical and academic impact of the fire.
"We have been overwhelmed by the offers of support from around the world since the fire and are hugely grateful to all those people who have donated.
"The support, along with that of the Scottish Government, has been vital.
"It has enabled us to put in place the Phoenix Bursaries programme to help those students who were most affected by the fire."
Mr Inns said the school was now beginning to look forward to the work which will need to be done to return the Mackintosh building to its former glory and an inspiring place for students to make work.
Glasgow born Peter Capaldi, who is well known for his portrayal of Malcolm Tucker in The Thick of It, studied graphic design at Glasgow School of Art.
He said: "I hope we can come together to help raise the funds to restore the Mackintosh Building to its former grace.
"It always seemed to me when I studied there both as a student and as a child in Saturday morning art classes, an exotic place of the imagination - all nooks and crannies, guarded by stern ancient statues and full of artistic possibilities.
"There is no greater symbol of the artistic spirit of Scotland than the Mackintosh Building.
"But more than that, it is a symbol of where art belongs, rising as it does out of the heart of a great city.
"A mighty castle on a hill, it is part of me and of all Glaswegians."
In a message to the students he said: "I am so sorry for all of you who have lost your degree shows.
"All I can say to you is that you are artists. This will only add to your story. You will be reborn and rise from the ashes even stronger."