The 28-year-old St Helens forward has inked a two-year deal and will link back up with head Dolphins coach Kristian Woolf.
The pair forged a successful liaison when Woolf was coach of St Helens and they won three consecutive Super League titles together.
"Morgan is one of the hardest-working players I have had the privilege to deal with," Woolf said.
""He is an international forward with an elite skill level and importantly, he is a winner.
"His four Super League titles and one Challenge Cup victory prove that he knows what it takes to win and is just the kind of player we want here at the Dolphins."
Knowles, who has played more than 200 games for Saints after debuting in 2015, is understood to have wrestled with his decision
He was born in Barrow-in-Furness in north-east England, and joined the St Helens scholarship program at 14 before progressing through their academy and signing his first professional contract with the club at 15.
Knowles will complete his Super League commitments with St Helens in 2025 in what is a huge year for him after the RFL granted him a testimonial season to recognise his contributions to the league.
He has represented Wales four times and England on eight occasions, most recently in last year's 2-0 series win over Samoa, where he impressed alongside Dolphins star centre Herbie Farnworth.
Knowles is the fourth signing announcement this week by the Dolphins as Woolf takes charge of his roster.
Utilities Max Plath and Kurt Donoghoe signed extensions, to 2028 and 2027 respectively. Newcastle prop Sebastian Su'a will also join the Dolphins from 2026 on a two-year deal.
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