Zac Brown is definitely out of the honeymoon phase -- 4 months after he and Kelly Yazdi tied the knot, they're now separated ... TMZ has learned.

Zac and Kelly tell TMZ in a joint statement, "We are in the process of divorce. Our mutual respect for one another remains. We wish each other the best and will always appreciate our time together. As we navigate this personal matter, we simply request privacy during this time.”

We broke the story that the pair secretly tied the knot on Aug. 31 in Coweta County, Georgia ... a little over a year after the model/actress was spotted wearing a giant engagement ring.

Speculation the newlyweds had split started when fans recently noticed Kelly had disappeared from IG ... and when she reactivated her account, she took Brown's name off her bio.

She also stopped following Zac on Insta, and took down all posts and photos that included him.

This was Zac's second stab at marriage -- he was previously married to Shelly Brown for 12 years until 2018, and they share 5 kids.