Using a box grater, grate pineapple into small pieces. Optionally, chop with a knife.
In a saucepan over medium heat, add grated pineapple, granulated sugar, water, lemon juice, and cornstarch. Stir until sugar has dissolved. Bring to a boil, then reduce heat to medium-low.
Simmer for 20 minutes, stirring occasionally, until mixture has reduced slightly. The jam should thickly coat the back of a spoon (and will continue to thicken as it cools).
Remove from heat, transfer to jars (if freezing, leave up to 1/2 inch of space at the top of your jar for expansion when frozen), and allow to cool completely before adding lids. Store for up to 1 year in freezer, or up to 3 weeks in the refrigerator.
If freezing, leave 1/2" at the top of the jar to allow the jam to expand slightly when freezing.
Freezer jam is NOT shelf stable and needs to be kept in the freezer or refrigerator. Treat your freezer jam like fresh produce, it will go bad after a few weeks in the refrigerator.