The singer, 34, posed for the cameras in the striking design on the red carpet at the star-studded London event.
Her look comprised of a long-sleeved crop and matching leggings, which she dressed up with a voluminous coat and heels.
She finished off her look with gold hoop earrings and a statement bag, and wore her hair slicked back in an elegant up-do.
Jessie announced last month that she was pregnant with her first child, just over a year after she revealed that she had experienced a miscarriage.
Shortly before her appearance at the Brits – which she hasn't attended for 10 years – she revealed to fans that she was expecting a baby boy.
Captioning a video of her singing on Instagram while holding her tummy, the singer wrote: "Since I wrote this song every time I play it or sing it he moves like crazy... so I think this song is the one."
She added: "Oh yeah... I'm having a boy."
Pixie Lott and Rochelle Humes were among those to comment their congratulations on the clip, which has received more than 422,000 'likes'.
However, Jessie wasn't the only star to make her baby bump the focus at the Brits on Saturday.
Love Island star Montana Brown, who also recently announced her pregnancy, opted for a cut-out black design.
Both stars appeared to have been inspired by Rihanna, who sparked the styling trend while pregnant in early 2022.
Watch: Pregnant Jessie J jokes she’s desperate for food as she arrives for BRITs