The 24-year-old French international halfback has inked a two-year deal from 2015.
“I’m pleased we’ve secured William as he will fit into one of our key positions on the field,” sadi London coach Joey Grima.
“He’s was impressive in the Dragons win over the Broncos in Round nine and led them around the field.
The player himself is looking forward to testing himself in England alongside some familiar faces.
“I’m looking forward to joining the Broncos as it will be good to learn a different system and to play with different players,” he said.
“I’m really motivated to be part of a winning culture that Joey Grima is creating, and I’m also looking to improve individually during my time at the club.