The former Real Madrid, Nottingham Forest and Tottenham Hotspur defender has had been employed in coaching roles at Dutch clubs Feyenoord and Excelsior.
New Pompey manager Tony Adams has been looking to the fill the vacant position since Joe Jordan left the club last week to join former manager Harry Redknapp at Tottenham Hotspur.
Metgod, who worked with Adams at De Kuip, told Feyenoord’s official website: “After all these years I leave here with pain in my heart but this is a challenge I cannot let pass by.
“Tony Adams is a key figure in British football but his thinking is influenced by Dutch football.
“We had excellent contact when he was at Feyenoord and that has remained.
“Tony called me on Friday morning to ask if I wanted to become first-team coach at Portsmouth.
“The chance to work with him and return to the highest level in England is appealing to me.
“As a player, I had four beautiful years there and I am looking forward to working in full stadiums every week and with the best players.”
Adams, who recorded his frst win as Pompey boss on Saturday when his side came from behind to beat Sunderland 2-1, was pleased to have got his man.
“John’s going to be a fantastic ally,” he said. “He’s the right person for this football club.
“He’s very calm and low-key. He’s going to support me and hopefully help the players.
“He’ll suggest players, he’ll work with the players. He jumped at the chance to come here.
“I’m delighted with my staff. We have good people in the right holes. If I need more staff I’ll add to it. If I don’t feel I need to, I won’t.”