The Everton manager feels the Brazil-born striker is vital to the world and European champions chances at the World Cup with a number of the squad set for their final tournament
Victory in Brazil will mark a second consecutive World Cup triumph and fourth at a major international tournament as La Roja look to become the first European nation to win in South America.
The Everton manager has pinpointed Brazil-born Costa to add "spice" to the world champions' 'Golden Generation' after struggling against the World Cup hosts in the Confederations Cup last summer.
"The individuals are superb and the way they play is very clear so youngsters can fit in," he told The Guardian.
"Now they have the extra element of Diego Costa, an ex-Brazilian who you can imagine is going to bring a lot of spice.
"Against Brazil last summer they needed to find a way to get out of their own half and they can add that to their play now with Costa. I do feel Spain is still the team to beat."
Martinez feels this summer's tournament is a "last chance" for a number of Spain's stalwarts to prove themselves on the world stage, with seven of Vicente del Bosque's provisional squad over 30 years of age.
"This generation is unique, the only team in world football to win three consecutive major international events. That has never happened before," he said.
"Its not just down to talent and a way of playing but to the mentality of the players. This is a generation that sees a last chance - players like Xabi Alonso, Xavi, Casillas.
"The group has a unique opportunity to be the first European team to win in South America. This team is moved by targets.
"They wanted to win the country’s first European Championships for years, then the first World Cup, then three tournaments in a row, Now they have the final target to be the first European team to win the World Cup in South America.
"The conditions are very different. The conditions will force Spain to find different solutions and the addition of Diego Costa can help that.
"As a team they will not accept changing because of what the opposition is doing on footballing terms, but I do believe the weather conditions and the South American way of playing means they will need more of an unknown quantity. Costa brings that."
Atletico Madrid star Diego Costa is a doubt for Saturday's Champions League final against Real Madrid with a hamstring concern, but is expected to be fit for the World Cup.