The South African has cost the Merseyside outfit R58-million (£4.5-m) and returns to Goodison Park after spending the second half of last season on loan there with the Toffees.
Pienaar moved to Everton in July 2007 on loan from Bundesliga giants Borussia Dortmund before signing on a permanent basis the following year.
He moved to Spurs in January 2011 but failed to establish himself under Harry Redknapp and subsequently rejoined the Toffees.
Speaking about his return to the club, he told evertontv: "I am happy to be here. It took a few weeks to sort out but I am delighted to be back and excited to play for Everton.
"There was no doubt in my mind that I would be back here. At the end of the season I told the manager how I felt and I am happy now it is all done."
Everton boss David Moyes had revealed earlier in the summer that his main target for the summer was to complete a deal for Pienaar.
The Toffees kick-off their Premier League campaign with a tricky home fixture against last season's runners-up Manchester United.