"Best #friends #no makeup #girl-time," McMillan captioned the fresh-faced picture.
Indeed, the 44-year-old star looks gorgeous -- her healthy skin glowing in the snapshot.
The former Friends star, who's engaged to actor Justin Theroux, has opened up before about preferring the natural look when it comes to her beauty regimen. Speaking to Marie Claire magazine in its July issue, the Living Proof haircare line founder explained, "For my wedding, I wouldn't do an updo, but I think a veil is always pretty. I like loose and natural hair because it goes against the formality of the dress. Loose, like, 'I just got out of bed, maybe had a romp' hair."
As for her flawless skin, Aniston attributed it her healthy, organic diet to New York Magazine in July.
"I can never stress enough to my friends that they must do as I do ... hydrate, hydrate, hydrate!" she told the mag. "Drink lots of water, get enough sleep, exercise, and eat a clean healthy diet, whenever possible."
"I think sometimes women may be overcleansing their skin. Some products and masks can be too aggressive and irritating for certain skin types," she added. "I believe the more simple, natural, and easy the skin care regime, the better off your skin will be."