In a day of light winds and overcast skies, the start was all-important and MAPFRE were away to a beauty on the 9.2-nautical mile (17-kilometre) course.
With Martinez at the helm, the red boat raced to the first mark with a narrow lead over Team Alvimedica (Turkey/US) and gradually stretched their advantage in a race completed in just under an hour.
The result leaves the Spanish crew still trailing the fleet in the overall standings on 31 points, but they have closed the gap considerably after a string of largely disappointing performances prior to Saturday.
Martinez, 37, was delighted to return to the winner's circle having returned to lead the crew after missing the previous leg due to Olympic commitments.
The helmsman from San Sebastian won gold in the 2004 Athens Games and silver in Beijing 2008, but finished disappointingly down the field in the 2012 London Olympics.
He is determined to put that right in Rio de Janeiro next year, but his commitment to his preparation has led to his missing several legs in this edition of the nine-month Ocean Race which started last October.
"We are improving slowly and that's the way we want to do these things," said Martinez after the victory. "Today we were first and we're feeling super good."
Team Alvimedica, led by Newport local Charlie Enright, clung on to second place while Abu Dhabi Ocean Racing stole third position from the all-women crew of Team SCA, who made a damaging positioning error mid-race.
The result puts Abu Dhabi, led by Briton Ian Walker, back on top of the overall in-port race series standings on 19 points. The series will count towards a separate trophy and be used to split ties in the main offshore competition.
The six boats in the fleet will begin the seventh leg, a 2,800-nautical mile (4,815km) trip to Lisbon, Portugal, on Sunday.
Abu Dhabi lead by six points from China's Dongfeng Race Team, with three legs to complete in the nine-month, 38,739-nautical mile (71,745km) marathon.
The race concludes on June 27 in Gothenburg, Sweden, having visited 11 ports and every continent.