Rinse the raspberries under cool water and gently pat them dry with a clean kitchen towel.
In a large, heavy-bottomed pot, combine the raspberries, sugar, and lemon juice. Stir gently to mix.
Place the pot over medium heat and bring the mixture to a boil, stirring occasionally with a wooden spoon to dissolve the sugar.
Once boiling, add the butter to reduce foaming and continue to cook, stirring frequently, until the mixture reaches 220°F on a candy thermometer, about 15-20 minutes.
Remove the pot from the heat and skim off any foam with a spoon. Let the jam cool slightly, then transfer it to sterilized jars, leaving 1/4 inch of headspace.
Seal the jars with lids and process them in a boiling water bath for 10 minutes to ensure preservation.
Remove the jars from the water bath and let them cool completely on a wire rack. Check the seals before storing.