Barker takes the reins at Fratton Park after Guy Whittingham was dismissed following an inauspicious start to the League Two season, with Portsmouth sitting in 17th position after 19 matches.
The 38-year-old - who enjoyed playing spells with Rotherham United and Mansfield Town - pens a rolling contract at the club, becoming available after parting ways with former club Crawley Town of League One on November 27.
Speaking about his appointment, Barker was thrilled with the chance to make a swift return to management.
"This is a fantastic opportunity for me," he told Portsmouth's official website. "I'm very excited to be at this club with its great history.
"Everyone knows how passionate the people of Portsmouth are about their football club."
Coppell - who had managerial spells at Crystal Palace, Manchester City and Reading - worked with Barker in the same role at Crawley and said he had no hesitation in accepting the job offer.
"It's a new challenge for me and I'm very excited," he commented. "Richie asked if I'd like to be involved and I jumped at the opportunity."
Barker's first task will be to arrest a five-match winless run in the league, and will have his first chance to claim three points in Saturday's home encounter with Newport County.
