Wash and prepare the fruit by chopping the rhubarb into small pieces and slicing the strawberries.
Sterilize your canning jars by boiling them for 10 minutes, using a bottle sterilizer, or heating them in a 275°F (135°C) oven for 10 minutes.
Boil the lids and rings in water for 10 minutes.
In a large, deep pot, combine the rhubarb, strawberries, sugar, and lemon juice.
Cook over medium-high heat, stirring frequently.
As the fruit softens, mash it gently using a potato masher until you reach your desired consistency.
Bring the mixture to a rolling boil.
Add the pectin and continue stirring.
While many pectin packages suggest boiling for 1 minute, boil the Strawberry Rhubarb Jam for 3–5 minutes to help achieve a better set.
Remove the pot from the heat.
Carefully ladle the hot jam into sterilized jars, leaving ¼ inch headspace.
Wipe the jar rims clean with a damp cloth.
Apply the lids and screw on the rings until fingertip tight.
Process the jars in a boiling water bath for 10 minutes, ensuring the water covers the jars by at least 1 inch.
Remove the jars carefully and place them upright on a clean towel.
Allow the jars to cool undisturbed for 24 hours.
Check the seals before storing. Refrigerate any jars that did not seal properly and use them first.