The pair caught up in New York City this week (4 December) as they indulged in some seafood at the Mermaid Oyster Bar.
Former Scrubs star Zach shared a funny photo of the pair embracing the moment, and grinning while holding up candles.
Captioning the snap, Zach said: "Romantic candlelit dinner with my BFF @Harry_Styles at Mermaid Oyster Bar in NYC."
Harry and Zach's bromance blossomed earlier this year when Zach found out that Harry started to follow him on Twitter.
Zach wrote in February: "Well Harry Styles follows me! And once he followed me, a zillion young girls followed me, like, whatever Harry chooses is the right way to go.
"So I'd like to thank Harry for following me because I got like a zillion 12 year olds who love me too. They think that me and Harry are hanging out together, going out on the town."
Harry and his bandmates - Zayn Malik, Louis Tomlinson, Liam Payne and Niall Horan - jetted into JFK airport earlier this week ahead of their performance on Saturday Night Live.
£2.49/month – that's all is costs for 4 issues of the REVEAL early edition. Gossip too good to wait for! Subscribe today!
Watch One Direction's music video for 'Story Of My Life' below: