South Africa captain Pienaar was forced to go under the knife after injuring himself in Spurs' final pre-season friendly against Athletic Bilbao on Saturday.
Down and out: Pienaar (left) will miss the start of Tottenham's campaign
The north London club said in a statement: 'Following scans and consultation with an independent groin specialist, Steven Pienaar underwent groin surgery this morning.
'The South Africa international was forced off with the injury in the first half of Saturday's pre-season friendly against Athletic Bilbao and is expected to be sidelined with the injury for up to six weeks.'
The 29-year-old has only featured 11 times since joining from Everton in January.
But with the future of Luka Modric still in doubt Spurs boss Harry Redknapp can ill afford to lose the midfielder on the eve of the new season.
Tottenham open their Premier League campaign against the Toffees at White Hart Lane before visiting Manchester United and hosting Manchester City in their next two matches.
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