"I believe that our future as a party and nation is bright and I have every intention of continuing to fight for our president, our party and our candidates," the Florida senator said in a statement.
His resignation came months earlier than anticipated. Martinez wasn't expected to step down until a Republican presidential nominee was selected, and the earliest that could occur is February.
The RNC said Martinez' job would not be filled.
Martinez, who is up for re-election in 2010, said he was relinquishing the job to spend more time focusing on his constituents and because the RNC had achieved the objective he set when he assumed the job in January.
"It was my goal as general chairman to lead the party as it established the structure and raised the resources necessary to support our presidential candidate and ensure Republican victories next November. I believe we have accomplished those goals," Martinez said.
The RNC has been the only national GOP party committee to outraise its Democratic counterpart this year. As of the end of August, the RNC had raised $55.3 million in contributions and had $16 million cash on hand. The Democratic National Committee had raised $34.8 million in contributions and had $4.7 million cash on hand. The DNC also reported a $2 million debt.
Martinez has shared the chairmanship with Mike Duncan, a longtime RNC official who has been responsible for the party's day-to-day operations. Republican officials say with Martinez' departure, the RNC will return to a traditional leadership structure with a single chairman.
President Bush named Martinez, a prominent Hispanic who previously served in the Cabinet, as general chairman last November.
He had been reluctant to assume the role and did so only after repeated White House overtures. When he accepted the job, he had indicated to friends that he anticipated serving only about a year in the post.
The first-term senator was brought on to be the face of the party, focusing on fundraising, outreach and travel to promote the GOP agenda.
In a statement, the president said Martinez "has effectively communicated our party's commitment to addressing the issues most important to all Americans. His message of hope and opportunity has resonated throughout America and strengthened support for our agenda."
Separately, Duncan called it an honor to serve with Martinez.
"Our party has effectively laid the groundwork for the 2008 Republican presidential nominee thanks in large part to Senator Martinez's efforts," Duncan said in a statement.
By tapping the Cuban-born Martinez to be the party's public persona a year ago, the White House had turned to a lawmaker who has been a staunch supporter of the president, including on the issue of comprehensive immigration reform, including a guest-worker program.
Martinez served as Mr. Bush's secretary of Housing and Urban Development from 2001 until 2003, when he resigned to run for the Senate seat left open by Democratic Sen. Bob Graham's retirement.
Martinez gained national recognition amid the drama surrounding Elian Gonzalez, the castaway boy who was returned from the U.S. to Cuba in 2000 after an international custody battle.
The top county official in Orange County, Fla., at the time, Martinez argued on national television talk shows and before a U.S. Senate committee that the boy should stay in the United States. He also invited Gonzalez to Walt Disney World before the boy was returned to Cuba.