The model and the Canadian singer, real name Abel Tesfaye, dated between 2015 and 2016, and after parting ways for a period, reunited earlier last year (18).
And to show their love for each other, the pair donned coordinating army print outfits to the party, which was held at Tao Downtown in New York City.
In a series of images posted on Instagram by Bella, the 22-year-old showed off her svelte figure in a green micro-mini skirt, cropped camouflage top and matching oversized jacket.
Bella accessorised with a pair of sheer Prada knee-high socks, heeled Timberland-style boots and hoop earrings.
Standing next to her in all of the sultry photos is the "Can't Feel My Face" hitmaker, who was wearing an oversized camouflage jacket with matching trousers, a dark shirt, black shoes and a chain around his neck that was engraved with the message, "Who God Has Blessed, No Man Can Curse."
"1 minute til my favourite day of the year... @theweeknd," Bella wrote beside one photo of the pair shortly before her boyfriend's special day began. "Happy birthday daddy," she cheekily captioned another shot.
The couple arrived at the bash in New York at around 1 A.M., and posed for many photos together, an onlooker told People. The singer was said to be very affectionate with his stunning beau, and could be seen holding Bella around her waist and kissing her.
Most of the guests, including Kourtney Kardashian's rumoured ex-boyfriend Luka Sabbat, Paris Hilton, Dapper Dan, Yolanda Hadid, Candice Swanepoel, Shanina Shaik, Jasmine Tookes, and Lameka Fox all followed the camouflage dress code, and posed for snaps at the party in front of a helicopter blade installation, according to Page Six.