Spaniard Muguruza was in the market for a new coach after splitting with Sam Sumyk in July.
The former world No.1 had Martinez in her corner when she won a second Major title at Wimbledon in 2017, when Sumyk took time out for the birth of his daughter.
Martínez was available after she recently finished working with Karolina Pliskova and the 1994 Wimbledon champion has now been employed by her compatriot Muguruza.
"Garbine and I will work together in the 2020 season. With new challenges and enthusiasm." Martinez tweeted.
Muguruza has dropped to No.35 in the rankings after winning just one title in Monterrey last season.