The news is another blow for the Ports who cannot sign players until next June after being found to have made undisclosed payments to Gary Twigg.
Portadown also had a £5,000 penalty imposed for the offence by the IFA.
Full-back Redman, who has spent 12 years at his hometown club, joins Crusaders-bound Michael Gault in quitting the Shamrock Park outfit.
Redman said that he had taken the decision "after a lot of thought".
"I have spent 12 years at Portadown and would like to thank the staff, fans and players at the club," said the defender.
"I have enjoyed every minute playing for me hometown club but feel that it is the right time of my career to move on. I wish everyone at Portadown FC the best in the future."
Ross Redman has announced that he is to leave Portadown after turning down the offer of a new one-year deal.