Businessman Todd Wilcox announced his decision after the Florida freshman senator decided on Wednesday to run for re-election.
"With all that is at stake, I have decided to end my campaign for US Senate and support Marco Rubio in his bid to keep this seat in the GOP," Wilcox tweeted.
Rubio thanked Wilcox, releasing a statement acknowledging his support.
"Todd Wilcox has dedicated his life to serving this country and his community," Rubio said
. "As a former Special Forces commander and CIA veteran, Todd brought a unique skill set to this race. I'm grateful for his support, and I look forward to working with him to protect the Senate and conservative principles."
The move leaves Rubio with only one primary challenger, businessmen Carlos Beruff, who issued a statement shortly after Wilcox's announcement.
"Now, the choice is clear. The voters of Florida can re-elect Washington's candidate, who has consistently failed to do the job they hired him to do and won't commit to serving a full six-year term. Or they can make a change," Beruff said.
Reps. Ron DeSantis and David Jolly decided to run for re-election to their seats in Congress rather than challenge Rubio, and the Florida senator's friend, Lt. Gov Carlos Lopez-Cantera, also got out of the way.
After the August primary, the Republican nominee will likely face off against Rep. Patrick Murphy, a favorite among establishment Democrats for the seat. Rep. Alan Grayson is also running for the Democratic nomination.