The 21-year-old will remain at the Tigers for the remainder of the season after the club beat the Super League signing deadline to secure his services.
"I have watched Michael play for a number of months now and I'm really pleased with his addition to the squad," said Castleford coach Daryl Powell.
"He's got fantastic footwork, good pace and he's a strong player who will continue to develop.
"He adds competition to the squad in the outside backs and we'll be looking to potentially tie up a longer-term deal with him."
Channing, who has targeted a place in Wales World Cup team, has made 12 appearances for the Broncos this term.