Basinas was a member of the Greece team which upset the odds by triumphing at UEFA EURO 2004 and Pompey chief executive Peter Storrie believes he is the ideal man to add some steel to manager Tony Adams's side.
"We've brought in someone of immense experience, the current captain of Greece who is just three games away from winning his 100th cap for his country," Storrie told Portsmouth's official website.
"He's exactly what we need in the situation that we're in. He's a player that Tony was eager to bring to Pompey.
"He desperately wanted to come here, and we have an option to sign him for a further year on top of the 18 months."
Pompey, last season's FA Cup winners, are just a point above the relegation zone after a dreadful run of results under Adams, who took over in October when Harry Redknapp joined top-flight rivals Tottenham Hotspur.
The south coast side have managed just one point from their last six games and went down 3-1 away to Fulham at the weekend.
During the January transfer window, they've seen key players Lassana Diarra and Jermain Defoe leave Fratton Park for Real Madrid and Spurs respectively.
However, they have held on to midfielder Sean Davis and Monday saw Pompey sign Basinas's compatriot Theofanis Gekas, a striker, on loan from German club Bayer Leverkusen until the end of the season.