The online activist group Anonymous said it took down the Trump Tower website on Friday.
The group said it made the move after it warned Trump about his statements on banning Muslims from entering the U.S.
Anonymous claims to be engaged in a cyberwar against the Islamic State.
Around 9 a.m. Friday, the Anonymous Twitter account @YourAnonNews posted tweets saying the group had taken down the Trump Tower site. The group also tweeted a link to a YouTube video in which it condemned Trump's recent statements calling for a ban on Muslims entering the U.S.
The Trump Tower website was unavailable during the early afternoon. Trump Tower is the flagship property of the Trump Organization.
Last week, Trump said the U.S. should not allow Muslims to enter the U.S. as a way to prevent terror attacks.