Prepare the fruit: Wash, hull, and chop the strawberries. Wash and trim the rhubarb, then dice into ½-inch pieces.
Combine ingredients: Add strawberries, rhubarb, lemon juice, and pectin to a large pot. Stir well and cook over medium-high heat until the fruit releases its juices.
Bring to a boil: Stir frequently until the mixture reaches a full rolling boil. Add the sugar all at once and stir until fully dissolved.
Cook the jam: Return to a rolling boil and cook according to your pectin package directions, usually about 1 minute, stirring constantly.
Test consistency: For extra reassurance, use the cold plate test. If the jam wrinkles slightly when pushed, it is ready.
Fill jars: Ladle hot jam into prepared jars, leaving ¼ inch headspace. Remove air bubbles, wipe rims clean, and add lids and bands.
Process if desired: For shelf-stable jam, process jars in a boiling water bath following safe canning guidelines. Let jars cool completely before storing.
