Rinse and pat dry raspberries, then transfer to a large pot.
Using your hands, crush the berries, then add sugar. Bring to a boil over medium heat, stirring with a wooden spoon, and cook until sugar has dissolved, about 5 minutes.
Reduce heat to medium-low and simmer, stirring often and skimming any foam that rises to the surface, until jam thickens and turns garnet, about 50 minutes. To check if jam is thick enough, spoon about 1 tbsp. of the hot jam onto a small plate and transfer to the freezer for 2 minutes. Then tilt plate; if jam "wrinkles," it's done.
Remove pot from heat and stir in lemon juice. Meanwhile, submerge 4 pint canning jars with their lids and ring bands and a wide-mouth funnel into a canning pot of boiling water over medium-high heat and sterilize for 10 minutes. Remove from hot water and transfer onto a kitchen towel.
Using the funnel, fill each jar with hot jam to 1⁄2" from the top. Wipe jar rims with a clean towel, place lids on jars, then screw on ring bands. Transfer filled jars to canning rack, then return to pot of gently boiling water for 5 minutes. Jars should be covered by at least 1" of water.
Carefully lift jars from water with jar tongs and place on a kitchen towel at least 1" apart to let cool undisturbed for 24 hours. Remove ring bands from cooled jam jars. To test that jars have properly sealed, press on center of each lid. When you remove your finger, if lid stays down it's sealed. Refrigerate any jam that hasn't sealed and use within 4 weeks.