Even A-list stars can have a frills-free summer afternoon!

Gwyneth Paltrow and hubby Chris Martin enjoyed a low-key lunch at the casual, laid-back Joni’s Kitchen in Montauk on Aug. 17.

We’re told the dressed-down duo and a few other pals dined outside at one of the spot’s picnic tables.

"They seemed like a normal group of couples enjoying Joni's," said a fellow diner, adding that Paltrow was "naturally-looking and beautiful," and appeared to be makeup-free.

We hear that the Oscar winner and the Coldplay frontman were in full view of the patrons lined up outside to place their orders (Joni’s doesn’t have waiter service), but that the pair was pretty much left alone.

Appropriately, Joni’s serves fresh, mostly organic foods, which health nut Paltrow surely loved.