蝶泳又名蝶式，或称海豚泳，是由蛙泳演变而来的。游蝶泳时俯卧在水面，两臂划水到大腿后提出水面然后从空中前移，手臂在头前入水完成一次划水循环，外形像蝴蝶，因而得名。为使游进更匀速，有人模仿海豚的波浪击水动作，大大提高了游进速度，因此又被称为“海豚泳”。蝶泳的速度和仰泳差不多，慢于爬泳，快于蛙泳。蝶泳需要很好的技巧和强健的肌肉，相比其他三种泳姿，对初学者来说最困难。蝶泳对于肱二头肌有一定的帮助，长期练习蝶泳的选手，骨架会稍宽。

The butterfly (colloquially among swimmers known as fly) is a swimming stroke swum on the breast, with both arms moving simultaneously, accompanied by the butterfly kick (also known as the "dolphin kick"). While other styles like the breaststroke, front crawl, or backstroke can be swum adequately by beginners, the butterfly is a more difficult stroke that requires good technique as well as strong muscles. It is the newest swimming style swum in competition, first swum in 1933 and originating out of breaststroke.