Paris Jackson's coffee run with her BF is stirring some buzz she might be engaged -- but a closer look has us thinking ... it's not all that serious yet.

17-year-old Paris and Chet Castellaw -- an 18-year-old soccer player for Real So Cal -- hit up a coffee shop in Calabasas on Saturday, and when they walked out photogs were laser focused on the ring finger of her left hand.

Yes, Paris and Chet have been posting a bunch of cute coupley pics on social media -- and, YES ... she is sporting a ring where engagement jewelry traditionally goes.

Now, the reality check -- we're not seeing any diamonds, and if you were proposing to Michael Jackson's daughter ... you'd deliver a little ice. No?

Just sayin'