The chart-topping Brit and Grammy-winning singer told the audience at Miami’s American Airlines Arena on Tuesday she did not want the Republican candidate to win.
In a video posted online, she is heard saying: “I do know what to do – don’t vote for him, that’s all I’m saying.”
She continues: “I support Hillary Clinton 100%, I do.
“And for anyone else that wants to vote for him, I don’t care, good for you, do whatever you want.
“I’m just letting you know who I would vote for.”
It is unclear if the Hello singer knew his bitter rival had dropped by the venue for a pre-birthday concert after attending a fundraiser.
He said she endorsed the Democratic candidate for the White House, telling fans: “I love her. She’s amazing.”
In February, Adele told Mr Trump to stop playing her music at his political rallies.
The billionaire businessman had been using the star’s songs such as Rolling In The Deep as “warm-up” music.
She is not the only musician to distance the property tycoon from her music.
In May, The Rolling Stones told the US presidential hopeful to stop using their songs at his campaign rallies.
Last week, rap superstar Eminem released a new politically-charged single called Campaign Speech, featuring a loaded lyrical attack on Mr Trump