Pompey manager Harry Redknapp has made Croatia captain Boris Zivkovic his third signing of the summer.
Zivkovic, 27, arrives on a free transfer from Bayer Leverkusen, having agreed a four-year £3m contract.
Pompey chairman Milan Mandaric said: "It shows the level of our ambition that we've signed the captain of Bayer Leverkusen, one of the top clubs in Europe.
"We're very excited that we're getting a world class defender. What is good about Boris is that he can play in a variety of positions across the back and also as a defensive player. I've made my manager a very happy man by signing Boris."
Zivkovic, who succeeded Robert Prosinecki as Croatia's skipper, is Pompey's third signing of the summer following the arrivals of Patrik Berger and Dejan Stefanovic.
Teddy Sheringham has verbally agreed to take a £25,000 per week pay cut to join Pompey on a one-year contract, but is yet to sign.
Meanwhile, the deals for Wimbledon duo Nigel Reo Coker and Jobi McAnuff have collapsed as the club's administrators are refusing to sanction the release of the two players.
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