"My warmest sympathy and best wishes goes out to the people of New Zealand after the horrible massacre in the Mosques. 49 innocent people have so senselessly died, with so many more seriously injured," Trump wrote. "The U.S. stands by New Zealand for anything we can do. God bless all!"
The White House also weighed on Friday and said the U.S. "strongly condemns" the attack.
"Our thoughts and prayers are with the victims and their families. We stand in solidarity with the people of New Zealand and their government against this vicious act of hate," press secretary Sarah Huckabee Sanders said.
Four people had been taken into custody over the attacks, according to New Zealand police. A man in his late twenties was charged with murder. The nations prime minister, Jacinda Ardern, denounced the massacre as a "terrorist attack."
The U.S. and New Zealand are partners through the "Five Eyes" intelligence sharing alliance, along with Australia, Canada and the United Kingdom.
CNBCs Everett Rosenfeld contributed to this report.