Justin Timberlake is just one big hopeless romantic! During his performance of 'Mirrors' last night at his tour stop in Sheffield, U.K., one fan was so moved that he popped the question to his girlfriend right there in front of the singer. JT caught the romantic act and announced the couple's engagement onstage, much to their surprise and delight.

Timberlake later paid tribute to the newly betrothed couple by posting their smiling photo on his Instagram account.

"Oh...And shout out to Chris & Serina, who got engaged during the show in Sheffield last night," JT captioned the above shot on Instagram, adding the hashtags, "#notabadthing #JT2020Tour."

This isn't the first time Timberlake has helped a couple become engaged on his 20/20 Tour. In December, he arranged the whole thing, inviting one lucky fan onstage to propose to his girlfriend in front of the entire crowd. The proposal was beyond romantic -- not only for the newly engaged couple, but also for the audience, who were undoubtedly swooning over the pure sweetness of the singer!

Of course, Timberlake also just premiered a new video/documentary for his song 'Not a Bad Thing,' searching for the couple who got engaged on the Long Island Rail Road to the hit. Sigh...

Needless to say, Jessica Biel is one lucky lady -- and our love for Justin Timberlake just keeps growing and growing.