The 22-year-old will join Cardiff after three years at the Gallagher Premiership club
The son of former Llanelli and Wales fly-half Colin, Stephens came through the system at Leeds Rhinos before switching codes in 2022.
The 22-year-old, who has previously represented England U20s, will add to the Arms Park side’s wing stocks, ahead of the departures of Owen Lane and Aled Summerhill.
“I’m really excited to join Cardiff. I love the style of play and feel it suits my attributes and abilities,” said Stephens.
“It will be a new city for me, a new environment and new league so I am really looking forward to that challenge and the opportunity to play with and against some of the best players in the world.
“I just want to play as much rugby as I possibly can, do my all for the team and show the coaches and everyone at the club what I can do.”
Speaking about the signing, Cardiff coach Matt Sherratt has backed Stephens, who can also play full-back, to fit into the Blue and Blacks’ gameplan as he builds his squad for next season.
“I’m pleased to bring Iwan to the Arms Park next season and excited to see what kind of impact he can have at the club,” said Sherratt.
“He is a ‘Cardiff’ type of player, who suits our style of play and has a bit of x-factor to create something out of nothing. He is a player who will excite our supporters and enjoy the fast pitch at Cardiff Arms Park.
“He gives us a bit more depth and versatility, and will compete with the likes of Josh Adams, Theo Cabango, Harri Millard and Gabriel Hamer-Webb for a spot on the wing.
“He is also Welsh-qualified, as the son of Colin Stephens, and will arrive at the club highly-motivated to prove himself in a new environment.”