Millie Bobby Brown Gushes Watching Boyfriend Jacob Sartorius Backstage

Millie Bobby Brown became the definition of "young love" while watching her pop star BF from backstage -- and after watching this ... you'll be saying AWWWWW.

TMZ obtained video of singer Jacob Sartorius' Wednesday night concert in Atlanta, where his new gal and fellow child star was perched behind a curtain looking on with a big fat smile.

There's no hiding it now -- Millie and Jacob are definitely a thing. The role of backstage girlfriend has been well established in relationships ... just ask Selena Gomez.