Kranjcar suffered the injury in training last week and missed Spurs' FA Cup semi-final defeat to Chelsea.
It had been hoped the versatile midfielder would be able to play some part in Tottenham's run-in, but Redknapp claimed that would not be possible this morning, telling the club's official website: "Niko is out for the season."
While not a first-team regular this season, the news is a blow to Redknapp as he was hoping to involve the 27-year-old towards the end of the campaign as Tottenham's midfielders have looked tired of late.
Kranjcar has made most of his 18 club appearances this season off the bench due to the form of Gareth Bale on the left wing.
Kranjcar, a 2009 signing from Portsmouth, found first-team opportunities limited last term and has only found the net once this season.
It is not known whether the midfielder will be able to take part in Euro 2012 for Croatia, who have been drawn in the same group as the Republic of Ireland.