In a large glass or glazed ceramic bowl, combine the chopped rhubarb, sugar, and fresh lemon juice. Stir well to evenly coat the rhubarb. Let sit for 8 hours.
In a nonreactive pot, bring to a simmer over medium-high heat and cook for about 20 minutes, stirring frequently. Lower the heat and cook for a further 10 minutes, stirring often.
Remove the pot from heat and allow the jam to cool for about 10 minutes. Spoon the warm jam into clean, sterilized glass jars. Seal the jars and refrigerate. Use within 2 weeks.