The 23-year-old, who made 93 appearances for the Pilgrims, has switched to a club nearer home following almost three seasons of mammoth commutes.
Marrs told bufc.co.uk: "I am sorry to leave because the fans have been brilliant, but this move makes sense in terms of where I am based.
"I wish everyone the best of luck for next season."
Marrs' only United goal arrived in January 2015 against Stockport County and he moves on with the best wishes of everybody at The Jakemans Stadium.