Hayden Panettiere -- That's GOTTA Be an Engagement Ring

Here comes the bride ... to-be ... at least, that's the way it seemed when Hayden Panettiere arrived to LAX yesterday with a HUGE DIAMOND on her finger.

Rumors have been swirling that the "Nashville" star finally said "Yes" to her massive heavyweight boyfriend Wladimir Klitschko ... but that bling on her hand pretty much put the rumors to bed.

So ... CONGRATS!!!!

Hayden was in a great mood on her way out of the airport yesterday (as usual) -- and there was only one topic to discuss ... weddings! Though it's unclear if she was referencing her own.

Check out the clip.