Children were among 22 people killed and dozens injured in the suspected suicide bombing on Monday night.
"So many young, beautiful, innocent people living and enjoying their lives murdered by evil losers," Trump said after meeting Palestinian president Mahmud Abbas in the occupied West Bank.
"I won't call them monsters because they would like that term. They would think that's a great name."
Trump added that "I will call them from now on losers because that's what they are. They're losers, and we'll have more of them. But they're losers -- just remember that."
Children were among 22 people killed and dozens injured in the suspected suicide bombing on Monday night at a pop concert by US star Ariana Grande in Britain's deadliest extremist attack in 12 years.
Trump is currently visiting Israel and the Palestinian territories as part of his first foreign trip since taking office. He held talks with Israeli Prime Minister Benjamin Netanyahu on Monday.