By Agence France-Presse
"This act of evil occurred as the victims and their families were in their place of worship. Our hearts are broken," Trump said on a visit to Japan.
"We pull together, we join arms ... through the tears and the sadness we stand strong," added the president, addressing business leaders on day two of his Asian trip.
A gunman stormed into a Baptist church during Sunday morning services and killed at least 26 people.
The shooter, whose motives were not immediately clear, was dressed all in black and wearing a ballistic vest, authorities said.
Though officials did not identify the gunman, multiple media outlets named him as Devin Patrick Kelley, 26, adding that he had been dishonorably discharged from the Air Force.
He also died in the assault.