The 18-year-old was a regular at right-back for David Hopkin's second string as they finished second in the reserve league last season.
A statement on the Largs website read: "The club are also pleased to announce the signing of William Boyd.
"William joins from Morton reserves."
Meanwhile, another Ton reserve defender has completed a permanent move to the junior ranks, after a successful loan spell.
Centre-half Euan Blair, who featured for the Cappielow club in pre-season friendlies against Rapid Bucharest and Campbeltown FP last summer, joined Hurlford United on loan in December.
The campaign was a successful one for the 17-year-old as the Ayrshire outfit won the West of Scotland Super League First Division and finished runners-up in the Champions Cup.
Blair said: "Just want to thank everyone at Morton who helped me over the last couple of years.
"I won't forget it."