Whittaker, a firm fans’ favourite after his loan spell with Argyle last season, becomes the Pilgrims' fifth summer signing ahead of the 2023/24 Sky Bet Championship season.
He has penned a four-year contract with the Pilgrims, and delighted Argyle boss Steven Schumacher said: "Morgan really needs no introduction to the Green Army after his contributions in our title-winning campaign last season.
"He scored and created crucial goals in the time he was with us, and we were all naturally disappointed when he was recalled in January.
"At just 22 he has so much room for growth and development, and we are thrilled he has decided that Home Park is the place he wants to do that. He gets fans off their seats and is one of the most exciting prospects in the EFL.
"He has been one of our main targets all summer, and it is a real coup to bring him back to the club. I know the fans will agree when I say I can’t wait to see him run out in an Argyle shirt once again.”
Director of Football Neil Dewsnip added: “We identified Morgan as a player we wanted to bring back to the club after his contributions last season and we are delighted to get this one over the line.
"He fits the profile of the player we like to work with, young and exciting, with loads of room for growth. We saw what he could do last year in League One, and there is no doubt in our minds that he will have a similar impact for us in the Championship this season.
"This is a club record deal for Argyle, and I want to thank Simon (Hallett) and the board for their continued support and faith in allowing us to bring in our top targets.”