Josh Duhamel's flying -- or rather, hiking -- solo with a backpack, cap and NO wedding ring.

Josh was spotted walking by his lonesome Thursday along a hiking trail in Brentwood. It's the first time the actor's been seen since news broke of his split with Fergie after 8 years of marriage.

As for Fergie ... she's down in Brazil fully owning the single life, with 4 new rings to replace the one Josh gave her. Now that the news is out, he's following suit with the rings off thing.

On Wednesday he was out with their son, Axl, and still wearing it.

What a difference a day makes.