The former One Direction hunk has shared his first new music in almost two years with the lead single from his as-yet untitled second solo album.
Harry has shared the video to the song, in which he is largely shirtless and grinds and dances with other scantily-clad people, with the whole group soaking wet.
Other shots in the video include the singer alone in a house and on the back of a motorcycle.
The track was produced by its co-writers, Tyler Johnson and Kid Harpoon.
Earlier this week, fans spotted billboards around the world which they suspected were connected with Harry's new music.
The huge posters, which were seen in London, Tokyo, New York, Los Angeles and Australia, bore the words 'Do You Know Who You Are', with the only clue they were promoting the 25-year-old star being his signature motto, TPWK - Treat People With Kindness - also spotted on the bottom.
And the question, "Do you know who you are?", forms part of the chorus on 'Lights Up'.
The 'Carolina' singer recently revealed he was "nervous" playing his new album for his friend Stevie Nicks.
He said: "It's a double-edged thing. You're always nervous when you are playing people music for the first time.
"You've heard it so much by this point, you forget that people haven't heard it before. It's hard to not feel like you've done what you've set out to do.
"You are happy with something and then someone who you respect so much and look up to is, like: 'I really like this.' "
Stevie has described the highly anticipated follow up LP as "spectacular", and she claimed it is a world away from the music he made with One Direction - and indeed from the material on his first standalone effort.
She said: "It's not like his last album. It's not like anything One Direction ever did. It's pure Harry, as Harry would say. He's made a very different record and it's spectacular."