The South Africa international returns to Merseyside a year on from his move to the capital after finding his first-team opportunities limited at White Hart Lane
The South African international left the Toffees last year to join Spurs in a £2 million deal but has struggled to break into the first team at White Hart Lane.
Everton chairman Bill Kenwright and Tottenham chairman Daniel Levy were discussing the deal for most of the day and negotiations advanced following the completion of Tuesday's Premier League games.
The deal to bring Pienaar back to the club where he made his name in England was sealed just before the 11pm deadline, with the player keen to thank his Spurs for allowing the move to happen.
Speaking via his Twitter, Pienaar said: "Very thankful that Spurs management allowed me to go on loan and I'm very happy it could be to Everton, a club close to my heart."
Pienaar has made just seven appearances for Harry Redknapp's side this season, scoring once in the Europa League win against Shamrock Rovers.
Before his move last year the 29-year-old had become a fans' favourite at Goodison Park after joining on loan from Borussia Dortmund in 2007 and then making his move permanent in 2008.
He played 130 times scoring 13 goals and will now look to add to his reptuation among the Toffees fans in his second spell at the club.