The celebrities have come together in a new campaign to share their memories about the day that changed the world.
The public service campaign is designed to inspire people to remember the 10th anniversary of the attacks with good deeds, voluntary work or charity donations.
Gaga, real name Stefani Germanotta, says in the clip on MTV.com: 'We were in disbelief. We all watched the second tower fall together.'
The event will take place across a variety of platforms and will continue until Sunday when a specially produced 'I Will' public service announcement will be aired.