Summerfield, who leaves Tranmere after spending two years with the League One side, becomes Rowett's first addition to the club since being given the position on a permanent basis last week.
The 53-year-old brings a wealth of experience to the Pirelli Stadium having worked as a number two at Sheffield Wednesday, Plymouth and Swindon, achieving promotion with all three clubs.
"I've looked at my own strengths and weaknesses and had to be honest about that and try to think of somebody who could complement those," said Rowett. "Kevin fits that bill; he has a wealth of experience and knows the league's very well. He's a good guy who I think the players will enjoy working with.
"He is very similar (to me) in his thinking on players and playing styles and we've built up a relationship."
Summerfield added: "The opportunity couldn't have come at a better time for myself. I've had two great years at Tranmere but I felt it was time for a fresh start. To me this is the perfect fit. Gary is a young upcoming manager. He has a lot to prove and I will be doing my utmost to help him."