He was speaking at the Republican National Convention in the US last night
Cruz finished second behind Trump in the race for the Republican nomination after their bitter rivalry during the campaign earlier this year.
Speaking at the convention in Cleveland Ohio yesterday, Cruz said: "We deserve leaders who stand for principle, who unite us all behind shared values, who cast aside anger for love.
"To those listening, please don't stay home in November. If you love our country, stand and speak and vote your conscience."
He added: "Vote for candidates up and down the ticket who you trust to defend our freedom and to be faithful to the Constitution."
In his speech Cruz backed some of Trump's policies, including the building of a wall along the US and Mexico border, but only mentioned the billionaire's name once.
He did heap criticism on Democrat Hillary Clinton and current US president Barack Obama, however.
Hillary Clinton believes govt should make virtually every choice in your life, but voters are overwhelmingly rejecting big govt #RNCinCLE— Ted Cruz (@tedcruz) July 21, 2016
He also mentioned Britain's Brexit vote, using it as an example of people rejecting big government and said that the June referendum was a "profound victory" and people were fed up with politicians who didn't listen to them.
Delegates eventually began to chant "Endorse Trump!" but resorted to booing shortly after when Cruz ignored their pleas.
Wow, Ted Cruz got booed off the stage, didn't honor the pledge! I saw his speech two hours early but let him speak anyway. No big deal!— Donald J. Trump (@realDonaldTrump) July 21, 2016
One delegate who has been heard shouting "Go home" at Cruz told Reuters: "It's taken me about 30 minutes to calm down and stop shaking with anger.
"I just don't know why Cruz did this. It baffles my mind."
In an unexpected twist and in a successful bid to deflect from Cruz's continued hostility, Donald Trump entered the arena to loud cheers and applause from the crowd. This immediately redirected attention firmly away from Cruz.
Trump will give his speech at the final event tonight.
Additional reporting by IRN