"I'm very excited to be the first female player to join with On," Swiatek - who previously competed wearing Japanese brand Asics - said in a statement on the On website.
The Swiss company, which started out developing running shoes, also announced a deal with 20-year-old player Ben Shelton.
The American made a name for himself by reaching the Australian Open quarter-finals in January on his first time outside the United States.
"Iga and Ben represent the next generation of world-class talent. Both players demonstrate On's competitive spirit and embody current and future champions of the sport," Federer said in the On statement.
Federer, who retired last year with 20 Grand Slam titles, invested in On in 2019 and designed its first pair of tennis shoes.
