"A year ago, I pledged to endorse the Republican nominee, and I am honoring that commitment. And if you don’t want to see a Hillary Clinton presidency, I encourage you to vote for him," the former Republican presidential candidate said in a lengthy statement.
Cruz, a senator from Texas who is a favorite of the conservative Tea Party movement, was one of Trump's last challengers for the Republican presidential nomination to drop out of the race.
When Cruz addressed the Republican National Convention in July in Cleveland, where Trump accepted the nomination, he declined to endorse Trump and was essentially booed off stage by the New York businessman's supporters.
During the heated primary battle, Trump had insulted Cruz's wife, Heidi, for her physical appearance and suggested that the senator's father was linked to President John F. Kennedy's assassin.
"Our country is in crisis. Hillary Clinton is manifestly unfit to be president, and her policies would harm millions of Americans. And Donald Trump is the only thing standing in her way," Cruz said.