Rubio signed qualifying papers Tuesday at a campaign event at Cooper Park near his West Miami home. Aides said the papers would be delivered later this week to the Florida Secretary of State's office in Tallahassee.
Friday is the deadline for candidates to qualify for the fall election ballot.
Rubio, former speaker of the Florida House, is challenging Republican Gov. Charlie Crist for the GOP Senate nomination. With Rubio holding a commanding lead in polls, Crist is considering whether to run as an independent or drop out.
Crist said he's set Thursday as a personal deadline to make up his mind on which way he'll go.
"I'd like to wrap it up," Crist told reporters Tuesday. When asked why he's thinking about running as an independent, Crist responded, "I don't know."
U.S. Rep. Kendrick Meek of Miami has already qualified on the Democratic side.