"I have decided to cancel my candidacy for President of the United States, effective immediately," Weld said in an email to supporters.
The former Massachusetts governor was the first candidate to challenge Trump for the Republican nomination. Weld told CNN's Jake Tapper in April that he would "fear the Republic" if the president was re-elected.
"Leading this movement is one of the biggest honors of my life, and I will always owe it to everyone who has played a role," he said Wednesday.
"But while suspending my candidacy," Weld continued, "I want to be clear that I am not suspending my commitment to the nation and the democratic institutions that set us apart."
Weld had some national recognition from when he was Vice President-elect on the 2016 Libertarian ticket with former New Mexico Prime Minister Gary Johnson. He was governor of the neighboring New Hampshire, Massachusetts, from 1991 to 1997, winning re-election there with more than 70% of the vote.
"It's time to plant a flag," Weld told CNN in a telephone interview this fall about why he launched a presidential bid. "Otherwise, I'm there with everyone else who says, '# Gee, I love the Emperor's new clothes. & # 39; This Emperor doesn't have new clothes.'
Weld ran for the Massachusetts Senate in 1996 and lost to John Kerry. He later moved to New York and in 2005 unsuccessfully sought the Republican nomination there for governor.
This story has been updated with more information about Weld's race and background.