The teen pin-up popped the question to the lucky lady who was dressed as a bride at the band's gig in Philadelphia after he picked her out of the sell-out crowd and joked he didn't want her efforts to go to waste.
While the boy band were thanking the audience at the end of their gig, Harry said: "I heard there was a girl here in a wedding dress ... is there a girl here in a wedding dress?
"Now I feel bad letting this go to waste so what's your name? Priya? Priya ... will you marry me? There's been a plot twist! OK .. awesome."
The 20-year-old singer then joked that he and his new love were going to wait until everyone had sent them presents to celebrate their engagement before calling it off.
He added: "We'll take the engagement gifts and then we'll ... OK, awesome. I almost wore my wedding dress. It could've been awkward. But that's a different story."
Priya was understandably beyond thrilled (despite receiving death threats from jealous fans), writing on Twitter: "I have completed all of my life goals as of today."
She later wrote: "Btw to all the people who were mean to me and threatened to kill me and spill soda on my dress.... I forgive you BC HARRY PROPOSED SUCKAS."
"I'm probably never going to sleep again to be honest. I just keep replaying "Priya will you marry me?" in my head IN HIS BRITISH ACCENT OMG "
While fans said: "Harry proposed to the girl in the wedding dress - that girl wins at life."
And: "Everyone that laughed at the girl who wore a wedding dress to the concert tonight. Harry proposed to her, who's laughing now."
Ladies man Harry has had several failed high-profile relationships with stars including Kendall Jenner, Caroline Flack and Taylor Swift.
Taylor appeared to make a dig at Harry - who she split from in January following a brier romance - at the Teen Choice Awards on Sunday when she made a cheeky quip while introducing the Choice Actor in a Drama Movie category.
Speaking to the audience, the 24-year-old beauty said: "Oh, hey guys. You know we've had some amazing memories here together at the Teen Choice Awards.
"And so, I was excited when they asked me to come and present this award, because no one knows more about dramatic males than I do."
Also, Harry has been criticised for wasting water
One Direction singer Harry Styles has been chastised by a water company which has accused him of encouraging fans to waste water.
Water firm United Utilities criticised a video he posted online, which shows the star brushing his teeth with the tap running.
"A running tap wastes more than six litres of water a minute," said a company spokesman.
Nobody from the band has so far been available for comment.
The firm, which provides water for north-west England, said the clip, which has received more than 1.3 million views, was a bad example to fans of the 20-year-old star, who grew up in Holmes Chapel, Cheshire.
Sean Robinson, spokesman for the water firm, said: "This video shows water heading in 'One Direction' - down the plughole."
United Utilities said it was commenting now on the clip, which was posted last year, because of the "ongoing debate about water efficiency".