Mum's the word for Mia! After being spotted at LAX on Monday with what looks like a diamond engagement ring.

Mia may not have eyebrows -- but she certainly has a ring on it!

While heading into the airport on Monday afternoon, the 21-year-old actress flashed a huge diamond ring on her engagement finger -- which is leading many people (including us!) to speculate that she and boyfriend Shia LaBeouf are engaged.

The actress wore a furry coat, white shirt, black pants and a pair or dirty white sneakers. She dodged our questions about whether or not the two were headed down the altar -- but we think that ring finger speaks volumes!

The twosome met in 2013 while filming Lars Von Trier's very controversial film, Nymphomaniac.