Kate Hudson -- On a Topless Beach! Punks Out (PHOTO)

Kate Hudson hitting a topless beach in Greece with her GFs sounds like the start of a beautiful thing -- and could have been ... if Kate understood how peer pressure's supposed to work.

Kate chose to keep her bikini top on, while her pals frolicked. It's unclear why she didn't let her girls run free -- maybe she spotted cameras nearby.

Hudson has been on vacation with her estranged hubby Matt Bellamy ... even though he's been hanging with a new blonde.

As for the beach -- we're pretty sure "When in Rome" applies, Kate. Even in Greece.