Morocco had led in the semi-final through a Soufiane Rahimi penalty towards the end of the first half at the Stade Velodrome in Marseille.
However, Barcelona’s Fermin Lopez equalised midway through the second period and then set up Sevilla right-back Sanchez to fire in the winner.
It is Spain’s second consecutive appearance in the final of the Olympic men’s football. They had to settle for silver three years ago in Tokyo after losing in extra time to Brazil.
Spain, who are chasing a second gold after triumphing in Barcelona in 1992, go to Paris for the final on Friday against either hosts France or Egypt, who play their last-four tie later.
Morocco can still claim a first medal by winning the bronze medal match, which will take place on Thursday in Nantes.
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