PD Photo: Uncropped picture of all of the figures in the famous "bikini girls" mosaic found by archeological excavation of the ancient Roman villa del Casale near Piazza Armerina in Sicily. This shows women exercising, running, or receiving the palm of victory and wreath for winning an athletic competition.
The mosaic also shows (at the lower left) a non-bikini figure, perhaps a games official, rushing in with a wreath and palm-leaf to crown the winner. Also note that these bikinis are not swimwear or swimsuits, but sportswear. But, the styling and fashion are also remarkably similar to some of the modern day swimwear and bikinis.