The chart-topping singer was announced as the new face of the popular fashion house earlier this year, so it's no surprise that she made sure to attend the birthday party during her time in New York on Monday (9 September).
And she couldn't resist getting stuck into the cake on offer.
Rita, 22, was caught pulling chunks out of the huge taxi shaped cake and stuffing the pieces into her mouth along with models Karlie Kloss and Joan Smalls and Australian rapper Iggy Azalea. However, we doubt they managed to demolish all of it.
The British pop star worked a smart-casual look for the evening, as she stepped out in a pair of long patterned shorts resembling a bandana and a matching top.
She teamed her quirky shorts with a white blazer, which was branded with the number '25' on the back, in tribute to the DKNY milestone.
Rita completed her outfit with a pair of pointy Louboutin heels and kept her short new hair 'do slick and straight.
Proving to be a big hit with brand creator Donna Karan, Rita was given the huge honour of closing the DKNY SS14 show at New York Fashion Week on Sunday (8 September).
She was seen strutting her stuff down the runway in a 90s inspired outfit to the Beastie Boys sound track, 'Fight For Your Right'.
What do you think of Rita's DKNY party outfit? Let us know below, as the best comment each week wins £50!