The decision, announced Thursday on Twitter, seems to mean Booker has chosen a possible race against U.S. Sen. Frank Lautenberg, a Democrat who is now 88, over one with Republican Gov. Chris Christie.
Booker's announcement alters the landscape for both races, and for politics in Newark, the state's largest city, where his term runs through June 2014.
"Let there be no doubt, I will complete my full second term as mayor," Booker said in a statement posted on Facebook and linked to on Twitter. "As for my political future, I will explore the possibility of running for the United States Senate in 2014."
It's not clear whether Lautenberg will retire or run against Booker, and probably others, in a Democratic Senate primary at age 90. A Lautenberg spokesman was not immediately available for comment.
Booker's statement did not address whether he would defer if Lautenberg decided to seek re-election. Booker, who was not made available to reporters to answer questions, praised Lautenberg and said he would "consult" with him. It's unusual for a candidate to announce plans to run nearly two years before a Senate election.
U.S. Rep. Frank Pallone indicated his continued interest in a Senate seat on Wednesday.
Booker reached out to Lautenberg on Thursday morning, but it's not clear whether the two talked. Booker also made a round of calls to Democratic county political chairs and state Senate President Stephen Sweeney, another Democrat said to be considering a run for governor or another office.
"It will be a privilege, an honor, to continue his legacy of service," Booker said of Lautenberg in a 3-minute video that accompanied his announcement.
Lautenberg, known as the lawmaker behind the ban on smoking on airplanes, served three terms in the Senate before retiring in 2001, but he returned to politics less than two years later, taking the ballot spot of scandal-plagued Sen. Robert Torricelli. In 2008, U.S. Rep. Rob Andrews took him on in a primary election, but Lautenberg won with the support of much of the state's Democratic Party establishment.
It gets touchier as he ages in part because the governor gets to decide who will complete an unexpired Senate term. As long as Christie remains in office, it's likely a Republican would replace Lautenberg should he not be able to continue serving.
While New Jersey voters have alternated between Republican and Democratic governors for decades, the state is solidly blue when it comes to the U.S. Senate. The last time a Republican was elected to represent the state there was in 1972.
Many Democrats viewed Booker as having the best chance at unseating Christie. Just one prominent Democrat, state Sen. Barbara Buono of Metuchen, has announced a gubernatorial candidacy so far. Now that Booker's out, the party will look for decisions from others including Sweeney and Sen. Dick Codey.
Buono on Thursday praised Booker's work in Newark and made a play for support from Democrats who were waiting to see whether he might run for governor.
"With the Mayor's announcement today - and having already earned the endorsement of the Middlesex County and Somerset County Democratic Parties," she said in a statement, "I am asking Democrats across New Jersey to join our campaign for the Democratic nomination for governor."
Assemblyman John Wisniewski, chairman of the New Jersey Democratic State Committee, who previously announced he would not run for governor next year, said he was disappointed that Booker isn't seeking the nomination.
"He's an attractive candidate," he said in a statement. "However, New Jersey Democrats have a number of talented, experienced individuals on our 'bench' who would make both excellent candidates and excellent governors."
Christie's popularity is at an all-time high following his handling of Hurricane Sandy and its aftermath. In announcing his re-election bid last month, Christie said he was motivated, in part, by the chance to lead New Jersey through the post-storm recovery, which he said won't be complete when his first term expires.
Booker and Christie historically have had a good working relationship. They agreed on the elimination of lifetime teacher tenure, for example, and on the need for government workers to pay more for their retirement and health benefits. The two even appeared in a "Seinfeld" parody video this year.
But Newark schools remain under state control and the city relies on the Christie administration for millions in aid to make up its deficit. As Booker eyed a gubernatorial run more publicly, Christie ramped up criticism of the mayor's fiscal management.
Critics say the celebrity mayor is much more popular beyond the city's borders than within Newark. Former Gov. Brendan Byrne, a fellow Democrat, cast doubt on Booker's ability to be as an effective an administrator as the governorship requires.
There is little doubt that Booker can raise money, however. He has brought in hundreds of millions in development and donations to the city, including a $100 million education donation from Facebook founder Mark Zuckerberg.
In the video, he touted other achievements of his administration, including more budget stability, court reforms and a reduction in crime.
"Amazingly, in a down economy, Newark is in its biggest period of economic growth since the 1950s," he said.
Despite the development and philanthropic money, Newark's unemployment rate stands at about 15 percent and the city continues to rely on the state to help balance the budget. The city council did not pass the 2012 budget until October. The state lent Newark $32 million last year, and it ended up with an $18 million surplus. So this year, Christie slashed the aid by $22 million, chiding the administration for how it handled the budget.
Booker has 1.3 million Twitter followers and is known for responding to constituent complaints sent to him electronically. During Superstorm Sandy, he invited residents to charge their cellphones at his house. In April, he let the world know through Twitter that he rushed into his neighbor's house and rescued her from a fire. During a snowstorm, he helped shovel people out. And he recently finished spending a week living on a food stamps-level food budget as part of a challenge that came from a Twitter follower.
He was elected mayor of Newark in 2006 with 72 percent of the vote, four years after narrowly losing a bruising battle against longtime Mayor Sharpe James. The race was chronicled in the 2005 documentary "Street Fight." He was re-elected in 2010 with about 60 percent of the vote.
A Stanford-educated Rhodes Scholar who grew up in suburban Harrington Park, N.J., Booker is the son of civil rights activists who were among the first black executives at IBM. He got his law degree from Yale Law School, then moved to one of Newark's most notoriously violent housing projects.
Associated Press Writers Katie Zezima in Newark and Geoff Mulvihill in Trenton contributed to this report.