Portsmouth's Chief Executive Peter Storrie told the clubs official website: "I am glad to have Tim here as I am never fully satisfied till everything is finalised.
"Tim is a box to box player and it does not need anyone to promote his record and how valuable he is to us.
"Harry Redknapp has long admired the player and our interest in adding him to our squad for some time has not particularly been a secret.
"We are very pleased to have overcome all the various hurdles and so announce that our perseverance has paid off with Tim becoming a Pompey player till the end of the current season.
"He is another player who knows what it takes to get to the top and stay there.
"At this stage of the season in our present position he is quite a catch," he added.
Sherwood has signed an initial contract that will keep him at Fratton Park until the end of the current season.