Harry Kewell, acclaimed as Australia's greatest-ever footballer, on Wednesday announced his retirement after 18 years in the game.

The former Leeds United, Liverpool and Galatasaray star said it was time to call it quits. He will play his last game for Melbourne Heart in Australia's A-League on April 12.

"I felt it was the right time to go out on my terms," the 35-year-old told a news conference.

Kewell, whose latter years were beset by injuries, said fitness had not played any part in his decision to hang up his boots.

"The body's great. I wanted to be able to finish where I could still do things outside of football, all the things I couldn't do as a footballer. I can do these things now," he said.

Kewell said it was a "privilege" to have played 18 years as a professional and "walk out not holding my back or my knees and still be able to push my body if I wanted to train".

He has not been part of the Australian setup in recent games and he said it was time for younger players to stand up and be counted at this year's World Cup in Brazil.

Kewell, who left home as a 15-year-old to play in Leeds United's youth team, became the Socceroos' youngest debutant against Chile in 1996 at the age of 17 years and seven months.

He went on to score 17 goals in 56 internationals and played in two consecutive World Cups -- Germany in 2006 and South Africa in 2010.

During his spell with Liverpool he won the FA Cup, the UEFA Champions League and UEFA Super Cup.

In 2012, Kewell was voted by fans and experts as Australia's greatest-ever footballer.

"It was an incredible honour and that's a title I will always remember and be extremely proud of," he said.

More than 15,000 public votes were cast during that selection process, while a panel of current and former players, administrators and commentators also helped determine the winner.