"I've always believed that. I still believe that. I'm proud I never compromised my faith in these principles during this campaign to score political points or tear down others. And maybe I'm stubborn, but I'll never abandon my faith in what we can accomplish when we join together."
Booker plans to run for re-election to the Senate, his campaign said.
"I will carry this fight forward - I just won't be doing it as a candidate for president this year," Booker wrote.
Booker is the latest candidate of colour to leave a Democratic field that had started out as historically diverse, exiting about two weeks after former HUD secretary and San Antonio mayor Julian Castro said he was suspending his campaign.
Booker, 50, had been seen a potential presidential candidate for most of his career. In 2002, a documentary crew followed his upstart run for mayor of Newark, New Jersey. Booker won that office in a landslide four years later, and was urged by Democrats to run for higher office, which he did by winning a special Senate election in 2013.
In the Senate, Booker had a liberal voting record, and took the lead on criminal justice reform legislation, like the First Step Act that passed at the end of 2018. As the only senator who lived in a struggling city - a fact he would frequently point out as a presidential candidate - Booker was an advocate for policies that would reduce incarceration rates and help poor families build wealth.
He was also friendly with the financial industry, at a time when Democratic voters were turning against it. In 2012, Booker criticised the Obama campaign's attacks on Republican presidential nominee's business career, calling it "nauseating." In 2017, he opposed an amendment to a budget bill that would have supported re-importing cheaper pharmaceuticals from Canada, a move that draw protests in his state, even though the amendment was largely for show.
As a presidential candidate, Booker was often stuck, unable to convince left-wing voters that he was on their side, while turning down donations and initially rejecting super PAC support that could have helped him.
He suffered from bad luck, too. After Booker criticised former vice-president Joe Biden for having touted his work with segregationists, the DNC's randomised rules put the two candidates on different nights of the June debates. California Senator Kamala Harris, who appeared alongside Biden, criticised those comments and briefly surged in the polls, as Booker stayed mired.
Democrats who watched the candidates were often surprised at Booker's lack of traction. He scooped up endorsements in early-voting states, and got some of the loudest responses at early multi-candidate events. Polls taken after debates consistently found voters rating Booker highly, and his favourable ratings rose in Iowa and New Hampshire.
Yet Booker, who had spent years building ties with black leaders, struggled badly with black voters. A hoped-for breakout in South Carolina never came, and in November, former Massachusetts Governor Deval Patrick entered the race, with Booker's weakness seeming to create an opening for a black candidate with executive experience.
Sagging in polls, Booker was cut from the December debate in Los Angeles, and joined other Democrats in criticising polling thresholds that several non-white candidates couldn't meet. His campaign raised US$6.6 million over the final months of the year, far less than the candidates who had made the stage.
"We started with one of the most diverse fields in our history, giving people pride, and it's a damn shame now that the only African American woman in this race, who has been speaking to issues that need to be brought up, is now no longer in it," Booker told MSNBC last month. "We're spiraling towards a debate stage that potentially [c]ould have six people with no diversity whatsoever."
When the DNC announced its line-up for this month's debate, that was exactly what had happened.
