The 23-year old Dutch-born Ghanaian international will join Pompey for the remainder of the season and is looking forward to trying to help the embattled club remain in the top flight with his return to the Premier League (he spent four years with Arsenal before joining Spartak).
"I'm happy to be here and hopefully I can have a big impact and help the club stay in the Premier League," he said.
"I've played in the Premier League before and it's great to be back," he added on Portsmouth's official website.
"I can't wait to get out on the pitch and play my first game for Pompey - that's what I'm waiting for.
"If we stay focused then I'm sure we can win some matches and get out of the relegation places."