More than 9,000 people got to their feet when the four-piece arrived on stage as one of the first acts to perform at the biggest pop event of the summer. The stadium erupted when Union J - featuring 20-year-old Josh Cuthbert, from Winkfield - bounded onto the stage, and performed tracks including debut single Carry You, which currently stands at number three on the iTunes download chart.
Also on the line-up was rapper Wiley, X Factor star Amelia Lily, boyband Lawson, and Charlie Brown.
Headliner Jessie J was the last act to perform, and plenty of excitable youngsters were eagerly awaiting her set.
Union J will next perform at the Capital FM Summertime Ball, in London, over the weekend.