Parkes, 20, spent four months on loan at Rovers last season and Pirates boss Mark McGhee has swooped to make him a permanent addition, subject to a medical.
McGhee said: "We are delighted to be bringing Tom to the club. He is a left-sided centre-half which will give us great balance playing alongside either Garry Kenneth or Adam Virgo.
"He proved last season that he can play as well, and he will be a great addition to strengthen our defence."
Chairman Nick Higgs added: "We are delighted to have agreed terms with Leicester to bring Tom to the club on a permanent basis.
"He was very influential last season, and we see him as a fantastic prospect for the future."