The Pilgrims boss has signed from Bury
Ryan, 40, comes to Home Park having guided Bury to promotion from Sky Bet League Two last season, his first full season in club management.
Ryan said: “I am thrilled to have been appointed Argyle manager and to have been given this opportunity to manage a great football club.
“I cannot wait to get started and to meet the legendary Green Army.”
Argyle chairman Simon Hallett said: “Ryan was the outstanding candidate from a very strong and impressive list of top-quality managers.
“We welcome him to Home Park, having no doubts that his footballing philosophy and attacking style is perfect for the way our supporters like to see Argyle play.”
Ryan will be assisted by fellow Liverpudlian Steve Schumacher, his right-hand man at Bury. Both will be introduced to the Green Army in due course.