The club confirmed the deal late on Tuesday night, shortly after striker Louis Saha moved in the opposition direction on a permanent deal.
The news caps a great day for the Merseyside club, who beat Premier League leaders Manchester City 1-0 and completed the signing of striker Nikica Jelavic from Rangers.
Pienaar, 29, left Goodison Park 12 months ago to sign a four-year deal at White Hart Lane but has failed to win a regular place in the Spurs line-up.
The move had been rumoured earlier in the day but appeared unlikely after Spurs boss Harry Redknapp revealed he had been unable to sign the player he wanted as cover.
But a deal was completed after a telephone conversation between Everton chairman Bill Kenwright and his Spurs counterpart Daniel Levy after their sides' respective games on Tuesday night.
The crucial paperwork was submitted just before the 11pm deadline and confirmation announced shortly afterwards.
Pienaar and Jelavic follow the arrivals of loan signing Landon Donovan and former Manchester United midfielder Darron Gibson at the Merseyside club earlier this month.
