In a large saucepan over medium heat combine cranberries and water; simmer for 10 minutes. Stir in mashed banana and sugar. Increase heat to medium-high; boil 1 minute. Stir in pectin and lemon juice.
Sterilize the jars and lids in boiling water for at least 5 minutes. Pack the jam into the hot, sterilized jars, filling the jars to within 1/4 inch of the top. Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids, and screw on rings.
Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil over high heat, then carefully lower the jars into the pot using a holder. Leave a 2 inch space between the jars.
Pour in more boiling water if necessary until the water level is at least 1 inch above the tops of the jars. Bring the water to a full boil, cover the pot, and process for 10 minutes.
