The couple — who collaborated together on the single Almost Is Never Enough and were seen cosying up in the music video — made the big announcement to fans on Twitter.
"So, I guess its obvious now…thanks to everyone who is being so lovely," 20-year-old Nathan wrote on Tuesday. "I'm so happy ❤️ #butweareterrified #pleasedontkillme #orariana."
His new girlfriend, also 20, followed his tweet, writing, "We hesitated saying anything as people can be a bit insensitive but thank u we value all of your happiness, thank you for valuing ours :)"
Sharing in the couple's joy was Nathan's Wanted bandmate Max George, who also took to Twitter to congratulate the pair.
"@NathanTheWanted @ArianaGrande very happy for you little brother," Max wrote. "She's a sweetheart ;)"
Nathan then replied, "@MaxTheWanted thanks mate, she really is."
News of Nathan and Ariana's romance won't have come as a complete surprise to fans. They were recently spotted holding hands and cuddling up during a visit to Disneyland, and were rumoured to have become close during filming for their music video.
Florida-born Ariana has not been shy in expressing her admiration for Nathan, particularly when speaking to MTV News about their duet.
"Nathan just sang the hell out of it," she said. "I thought it was Brian McKnight. I'm like, 'Who is this?' Honestly I wasn't expecting it. I didn't know he had that in him.
"I saw him perform live on one of the radio shows that we were at together, and I was like, 'Damn, he can sing.'
This song I think will pleasantly surprise a lot of people. It's very beautiful and very emotional and I love, love, love what Nathan did to it. It was a great, grew, great surprise."