Charli XCX Poses Topless on River Thames For Playboy

Charli XCX brought Brat Summer to the water for Playboy ... stripping down and posing topless while riding a barge down a famous waterway in the United Kingdom.

The singer-songwriter posed for the iconic magazine in a cover story published Wednesday ... putting her body on display in a hotel room -- black bra barely covering her chest -- before taking the shoot on the road. Or, more accurately, the water.

On the boat, Charli strips the top off completely ... though she tastefully covers her boobs with her hands while posing in front of notable London landmarks along the River Thames.

Charli didn't just open up her shirt ... she opened up her heart ... discussing her marriage to George Daniel ... and admitting she's got the hots for people who have the toxic trait of being "a tiny bit avoidant."