Katsouranis made his international debut for Greece on 20 August 2003 against Sweden and scored his first goal against Kazakhstan in a 2006 FIFA World Cup qualifier. Katsouranis was one of the key factors in Greece’s triumph at UEFA Euro 2004, beating Portugal in his own “home” (Benfica’s Estádio da Luz) and, following Greece’s success at the tournament. Forced his way into the starting XI after coming off the bench for the opening game of the tournament, he was an extremely important part of a three-man midfield that also included Theodoros Zagorakis and Angelos Basinas. It was in Portugal that Katsouranis really made a name for himself, delivering a string of consistent performances culminating with that famous final in Lisbon. Lifting the Henri Delaunay trophy represents the pinnacle of a national-team career that so far has never again reached the same heights.

Katsouranis signed a four-year contract with Portuguese club Benfica on 22 June 2006, joining his former manager Fernando Santos and fellow countryman Giorgos Karagounis. When he arrived in June, he said “Even if Fernando Santos or Karagounis were not here, I still would have joined. Benfica to me is one of Europe’s top clubs and they proved that in the UEFA Champions League. I am here to achieve that same objective and joining Benfica can also help my international career.”

Despite interest from Valencia CF, Werder Bremen, Tottenham Hotspur, and Juventus, Benfica refused to sell the player, despite Juventus willing to trade Tiago for the Greek international. Benfica were reluctant to sell and, on 14 September 2007, Katsouranis agreed to a contract extension with Benfica for another two years. During his last season there, he was given a chance to captain the side, which was a huge honour.

On 2 March 2009, Katsouranis was named “Benfica Player of the Year”.

Katsouranis signed a four-year contract with Greek Super League club Panathinaikos, on 1 July 2009 on a fee of €3.5 million. He has stated that it’s the team he supported since childhood. He scored his first goal against Sparta Prague in the Champions League. He scored his first league goal against Skoda Xanthi on the 2nd matchday of the 2009–10 season. Katsouranis scoring abilities were a key part of Panathinaikos’ good run during the first half of the season, as the defensive midfielder scored 8 goals in just half a season. He ended the season with Panathinaikos managed to end the season positively by winning the double with the 2009–10 Greek Cup final against Aris Thessaloniki. Katsouranis ended his long wait for his first trophy by helping the Greens win the domestic double in his first season; was the team’s second top scorer (behind Djibril Cissé) with eight goals in the Superleague Greece triumph.

Katsouranis became one of Greece’s impact players in their UEFA Euro 2008 qualifiers in which he helped Greece qualify for the finals and have a chance to retain their crown. Their UEFA Euro 2008 campaign resulted in poor fashion as Greece failed to pick up a point. Katsouranis has captained Greece on several occasions and was selected for his country in the 2010 FIFA World Cup qualification. Together with Angelos Charisteas, Giourkas Seitaridis, Georgios Karagounis, Kostas Chalkias who all participated in Greece’s UEFA Euro 2004 triumph, he led Greece to a 2–1 win against Nigeria, its first win in a World Cup tournament.

In October 2012, Panathinaikos president Giannis Alafouzos terminated his contract with the club, with Katsouranis paying the price for Tryfili’s bad appearances at the start of the 2012–13 season. Katsouranis has been serving a six-match suspension for verbally abusing a referee in a Greek Super League match in August. Alafouzos reportedly told the rest of the squad that the player was “history” at Panathinaikos, whilst also expressing his displeasure about “two or three other[s]”, without going into specifics.

Katsouranis encounters that gave and took many from the club. When signed with PAOK in December 2012, was not in the best moment of his career. He left Panathinaikos with rumours for a lot of problems with foreign players and the category of “the boss in changing rooms”. Katsouranis never replied to a series of miseries which were released and related to him. He said “yes” to PAOK, not only because he wanted to prove that it can be helpful to club plans to become a champion, but also because he wanted to make it clear that working with people who trust him and with a serious administration, he will never be a problem for those who want to hide in the seatbacks and ask severe criticism. He succeeded in both. In fact few people know that he went to PAOK without the blessing of his former coach Giorgos Donis and needed only a few weeks to become one of his favourites.

Reports from Thessaloniki suggest that the defensive midfielder, who ended his contract with Panathinaikos by mutual consent in October 2012, will get €700,000 for this year and a half, plus any bonuses. It forms part of the policy of the new PAOK owner, Ivan Savvidis, who insisted on the acquisition of Greek internationals in order to boost both the strength of the roster and the prestige of the club.

On 11 December 2012, he signed a contract until the end of the 2013–14 season with PAOK and made his club debut on 6 January 2013 against Panthrakikos. He would wear number 28 on his jersey. On 24 January, he scored his first goal with PAOK in the Greek Cup against Kallithea. On 3 March 2013 Katsouranis scored his first goal in the Greek Super League with PAOK in a 4-2 win against Panionios. He formed a strong partnership with Gordon Schildenfeld in the central defence reducing to a great extent the goals which PAOK conceded. On 27 August 2013 Katsouranis scored against Schalke 04 for the Champions League playoff in a 2–3 loss.

On 17 October 2012, Katsouranis reached a milestone on the international level as he made his 100th appearance in a Greece shirt. A 1–0 win over Slovakia in Bratislava certainly stole the headlines, but Katsouranis became just the fourth Greece player to make a century of appearances for Greece just days after teammate Giorgos Karagounis set the all-time Greece record for appearances.

On 27 February 2014, he was sent off in the 69th minute in a 3–0 UEFA Europa League loss against Benfica, and he received a standing ovation from the Benfica fans as he exited the field in recognition for the seasons he spent with Benfica.

On 16 April 2014, in an episodic semi-final game for the Greek Cup, where fans of Greek sides Olympiacos and PAOK put the mental in monumental as they greeted their heroes with a ring of fire, he had a key role in helping PAOK eventually went through on away goals after a 1–0 win following a 2–1 defeat in the first leg in Athens. It was a tremendous game as the fans then use the pitch as a giant ashtray by throwing their flares onto the grass and set fire to one of the dug outs. Five minutes before the end of the game for the sake of a ball he falling out with his team player in the Greek National Football Team, Giannis Maniatis leading to his drop-off from the game.

On 26 December 2014, he signed a six-month contract with Atromitos with an option to extend it by one year. Undoubtedly he was the most impressive transfer of this winter’s window, the signing of former Greece skipper. “I will give it all for Atromitos”, stated Katsouranis who in the first half of the season plied his trade in India, with remarkable success in that up and coming mini-league that lasts only a few months. The 35-year-old has made no secret of his aspiration to return to the national team, too, as former Greece manager Claudio Ranieri had not called him up for any of the five matches Greece played after the 2014 World Cup. On 3 January 2015, he played the first match with the new club at home against Kalloni. He signed for Atromitos in a six-month deal with an option for one year more, however, the Athens club decided not to keep Katsouranis in their squad ahead of next season.

Katsouranis’s exceptional leader characteristics along with experience and the huge influence on the pitch saw him stand-in as captain for Greece at the 2014 FIFA World Cup, with the team’s regular captain, the veteran Karagounis, restricted to substitute appearances at the age of 37. On 19 June 2014, Katsouranis became the first Greek player in World Cup history to be shown a red card. He was dismissed in the 38th minute of the group match against Japan for committing two bookable offences.

On 15 September 2015, Katsouranis signed with NPL Victoria club Heidelberg United on a one-match contract, with the option to extend it should they win their following FFA Cup match. Heidelberg United coach George Katsakis summed it up by commenting: “When you start to put things into perspective, he played for S.L. Benfica, all over the world, to come to the Olympic Village for Heidelberg, it’s amazing.”

On 29 September 2015, Katsouranis officially announced his retirement from professional football.

On August 2016, after his retirement from professional football, has appointed as a Technical Director of Football League club Panachaiki.

On 14 November 2017, Katsouranis was appointed as Ambassador of the United Nations. The announcement stated: “The European Champion and Technical Director of Panachaiki F.C. Kostas Katsouranis was delighted to participate in the information campaign on the 17 UN Sustainable Development Goals. Sport and Athletes around the world play a very important role in achieving the Goals, Development, Peace and Human Rights. We like to thank Kostas Katsouranis for his participation and his substantial contribution to the success of their efforts.” On 28 November 2018, he resigned from the position.