Redknapp replaced Juande Ramos, who was sacked a day earlier with Spurs bottom of the Premier League.
The north London club agreed a compensation package of around £5million with Portsmouth, who had won the FA Cup under Redknapp the previous season.
Redknapp said: “I suddenly got a phone call and was told Tottenham were interested in me, and I thought it was a wind-up at first.
“Then when I had spoken to them, I decided it was a good move for everybody, for Portsmouth as well in the long term and they have a terrific compensation deal when things were a bit tight.
“I just thought maybe it was time to move on and Tottenham are a big, big club.”
Redknapp had an immediate impact in the dugout, winning five of his first six games in charge – the other result a 4-4 draw at Arsenal were the visitors struck twice in the last two minutes to rescue a point.
Spurs also reached the League Cup final but lost on penalties to Manchester United and by the end of his first full season Redknapp had led Tottenham into the Champions League.
Fifth and fourth-placed finishes would follow in the next two years with players such as Gareth Bale, Rafael van der Vaart and Luka Modric starring.
Despite the consistency, Redknapp was sacked in June 2012 having also been overlooked for the England job which was filled by Roy Hodgson.
REUTERS