The 24-year-old artist was spotted at the event in London, which was in collaboration with Sink the Pink.
He showed off his even slimmer body whilst posing for photographs at the night out.
He admitted to losing 14 pounds in just 14 days in 2015 which started his weight loss journey.
The Stay With Me singer accredited the diet to nutritional therapist America Freer, who has worked with other stars such as James Corden.
He spoke out about the effects, after losing a huge three stone during the journey with the diet.
He posted on Instagram: “Amelia Freer has helped me lose over a stone in two weeks and has completely transformed my relationship with food.
“Everyone go check out her incredible book, and start to live healthy. It’s not even about weight loss it’s about feeling happy in yourself.”
The book became a best seller overnight after his comments advocating the health plan.
Other stars have praised him for his amazing weight loss over the years.
Taylor Swift was one who spoke out about his transformation.
She stated: “He’s always been a babe. He’s getting fitter, he looks great but I always thought that Sam looked great.
“I don’t really care how much he is working out or whatever, I think it’s those eyes.”