Martinez has signed on for what is believed to be a four year deal – one year longer than the deal he had beforehand.
"We have agreed a new, improved contract with Roberto that extends beyond the length of his current deal." said Chairman Huw Jenkins
"After a fantastic season that has seen Roberto guide the club into the Championship for the first time in 24 years, we are delighted to announce that he has committed his long-term future to Swansea City."
Martinez told BBC Radio Wales: "I feel flattered. The long relationship has always been in my mind.
"The contract is for as long as we can work together in terms of fulfilling each other's ambitions."
He added: "We have come a long way in the last 10 months and we really believe that the next three or four years are going to be vital for our football club.
"In that respect, we are ready for the next challenge and to be rewarded with this is special."