The 57-year-old quit Pompey after chairman Milan Mandaric brought in executive director Velimir Zajec, and his move was described by Fratton Park director Terry Brady as "the highest betrayal possible".
Redknapp expressed his delight to be taking over the Saints and insisted he felt Pompey fans would accept his move.
He said: "I have always regarded Southampton as a very well-run club and I am delighted to be joining them.
"I can't think of a similar situation on the south coast but I don't see a problem with the vast majority of either fans. April 23 (when the two sides meet at Fratton Park) will be a cracker."
But Redknapp's much-anticipated return to his former club fell flat, with the home side charging to a 4-1 win which distinctly took any remaining sting out of the situation.
Redknapp locked his players in the dressing room for 45 minutes after the result, and made good on his insistence that he would not fulfil post-match media duties whatever the result.