The actor - who convention officials have been referring to as a "mystery guest" - is scheduled to speak just before Florida Sen. Marco Rubio in the 10 p.m. ET hour. Mitt Romney will accept the Republican presidential nomination after Rubio speaks.
Eastwood endorsed Romney in early August at an Idaho fundraiser, stating that "the country needs a boost."
Eastwood's appearance during a "halftime in America" Super Bowl ad in February was taken by some as an endorsement of President Obama. Eastwood said at the time he was not affiliated with a candidate.