Stir rhubarb, sugar, ginger, and lemon juice in a large pot and allow to stand until rhubarb releases its juice, about 20 minutes.
Place over medium-high heat and bring to a boil; cook until thickened, about 15 minutes, stirring often.
Sterilize the jars, lids, and rings in boiling water for at least 5 minutes. Pack the jam into the sterilized jars, filling them 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, 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.
Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several inches apart, until cool. Once cool, press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all).
