Begin by washing and chopping fresh rhubarb into small pieces. Combine rhubarb, sugar, lemon juice, and cinnamon in a pot and let sit for 10-15 minutes.

Place the pot over medium heat, stirring to dissolve the sugar completely until the mixture is juicy and bubbly, about 5–7 minutes.

Once bubbling, sprinkle in the pectin while stirring continuously to prevent clumping.

Bring the mixture to a full rolling boil over medium-high heat and stir for 1 minute.

Carefully pour the hot jam into sterilized jars, leaving 1/4 inch headspace, and wipe rims clean.

Process jars in a boiling water bath for 10 minutes to ensure sealing.

Let jars sit for 5 minutes in hot water, then cool on a towel or wire rack.