Remove the skin and core of the pineapple and cut into thin spears.
Arrange the spears in the mason jar, filling it until it's about ¾ full.
Pour the tequila into the jar until it covers the pineapple. Seal and refrigerate until infused.
When ready to use, strain the tequila, discard pineapple and use in your favorite margarita recipe or sip over crushed ice.