AP Photo/Chuck Burton
Nikki Haley and Marco Rubio.
Just three days before Saturday's primary, Haley is scheduled to appear with Rubio at a 6 p.m. town hall and announce her endorsement of the senator, The Post and Courier reported.
Sen. Tim Scott (R-South Carolina) and Rep. Trey Gowdy (R-South Carolina) previously endorsed Rubio.
"I would love to have her endorsement," Rubio said in a previous meeting with The Post and Courier's editorial board. "I'm not sure she's going to endorse anyone."
Fellow candidate and former Florida Gov. Jeb Bush (R) told NBC News in an interview prior to Haley's decision that her endorsement "would be the most powerful meaningful one in the state."
He added that, were he not to get Haley's endorsement, it would send "a signal that I got to work harder."
Haley, a rising star in the party who delivered the GOP's response to President Barack Obama's State of the Union speech earlier this year, previously said she was unsure if she'd endorse any of the candidates, although she had been tough on GOP front-runner Donald Trump.
On Tuesday, she said Trump represented "everything a governor doesn't want in a president." And, in her State of the Union response last month, alluded to Trump when she said, "During anxious times, it can be tempting to follow the siren call of the angriest voices. We must resist that temptation."
Rubio is currently polling third in South Carolina at 16%, according to the RealClearPolitics average of several polls. He trails Sen. Ted Cruz (R-Texas), who is at 18%, and Trump, who is at 35%.