Coming days before the South Carolina primary, the endorsement would be a major blow to former Florida Gov. Jeb Bush, who had hoped to secure Haley's backing.
Haley has emerged as a major voice in the state and nationally in the Republican Party, winning bipartisan praise for her handling of the massacre at a historic African-American church in Charleston last summer and subsequent debate over removing a Confederate flag at the statehouse.
Haley had made no secret that she did not plan to endorse Trump, but it was an open question whether she would back Rubio or Bush.
Bush added if he didn't get the endorsement, "It sends a signal that I got to work harder."
Haley endorsed Mitt Romney in the 2012 GOP presidential race. Romney lost in South Carolina to Newt Gingrich that year, though he ended up winning the Republican nomination.
--This report was updated at 2:29 p.m.