Keane, who was sold by Spurs for 19 million pounds ($26.90 million) last July after handing in a transfer request, joins former Tottenham team mate Jermain Defoe after he also returned to the struggling north London club from Portsmouth last month.
"We are delighted to announce that we have reached agreement for the transfer of Robbie Keane back to the club from Liverpool," a statement on Tottenham's website said. "Personal terms have been agreed and a medical completed."
Neither Premier League club disclosed the transfer fee.
With French defender Pascal Chimbonda also signing for Tottenham from Sunderland in January, Spurs have bought back three players sold when previous manager Juande Ramos was in charge at White Hart Lane.
Keane made 28 appearances for Liverpool, scoring seven goals, but struggled to hold down a regular place in Rafael Benitez's starting side. He admitted on Monday his "lifelong dream" of playing for Liverpool had not worked out.
"It was a difficult decision to make to leave Tottenham in the summer," Keane told the Spurs website. "It proved not to be the right move for me.
"I know some Spurs fans will feel I let them down by leaving but I can assure them I shall be giving my all for this club (which) has terrific fans and I want to repay them for all their support. We've got to get on and fight our way up that league table."
It is a surprising U-turn for Keane and another twist in the career of the 28-year-old who began at Wolverhampton Wanderers and had spells with Coventry City, Inter Milan and Leeds United before joining Tottenham in August, 2002.
Spurs chairman Daniel Levy said: "We never wanted Robbie to leave in the summer, we acknowledged the six years dedicated service he gave this club and reluctantly let him go. I am delighted that he has chosen to return to us."
Keane, Ireland's record scorer, became a firm favourite with Tottenham fans, netting 107 goals in 254 games in his first spell there. He struck up a lethal partnership with Bulgarian Dimitar Berbatov who was sold to Manchester United in August.
Spurs manager Harry Redknapp, who replaced Ramos in October after a terrible start to the season, will hope the return of Keane, often the captain at Tottenham, can help lift them away from the relegation zone.
Russian signing Roman Pavlyuchenko and Darren Bent have failed to fill the void left by the sales of Berbatov and Keane this season with Spurs having scored only 11 goals in 12 home league matches.
They are 14th in the table but just two points above bottom club West Bromwich Albion
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