The 24-year-old has been captaining Leicestershire this summer but, along with several other members of the Grace Road squad, has decided to develop his career elsewhere.
It is a part of the County’s recruitment scheme that has been a long time in the discussion process.
But now it is all completed, Northamptonshire chief executive David Smith is a very happy man.
“We are very happy to have brought Josh here,” he said.
“He was David Ripley’s number one target for us to sign so to have been successful with that is very pleasing.
“He will bring a cutting edge to our white-ball cricket as he is a class act in that form of the game.
“He was the man on the match in the 2011 T20 final, for example.
“He is also a gun fielder and someone who can bowl some tidy overs as well.
“On top of that he brings captaincy experience with him.
“As someone who has just turned 24 he has his best days ahead of him and he could offer a lot to this club for several years to come.
“He’s very fit, has had a good Championship season and is a very decent player to have coming into strengthen the club.”
Cobb said: “I want to firstly thank Leicestershire and all the fans for all they have done for me.
“Captaining my boyhood team was an honour and something I will always cherish.
“But I feel it’s now time to start a new chapter and challenge myself to improve in a new environment.
“I want to wish Leicestershire all the best for the future, Grace Road will always be a special place for me.
“I’m very ambitious both personally and to be a part of a winning team.
“I want to play at the highest level and play the best domestic cricket available.
“Northamptonshire have proven that they are a winning limited-overs team and have set clear targets to get back into Division One next year.
“I want to be a part of this and after talking to the management, who I know well, I’m convinced it’s the right place for me to achieve my goals”.