Wash your jars, lids, and bands in hot soapy water, rinse well, and dry. Set them aside on a clean towel.

In a large, heavy-based pot, add raspberries, sugar, and lemon juice. Gently stir to mix, and let the mixture sit at room temperature for 20 minutes to melt some of the sugar and bring out the berry juices.

Place the pot over medium-high heat and bring the mixture to a rolling boil. Stir often and skim off any foam that forms. Continue boiling for about 10-15 minutes until the jam thickens and reaches 220°F on a candy thermometer.

To test if the jam is set, drop a small amount onto a chilled plate. If it wrinkles with a light push from your finger, it is ready. Remove the pot from heat, let it cool for 10 minutes, then fill your sterilized jars while the jam is still warm. Seal the jars immediately.

Allow the filled jars to cool completely at room temperature. Once cool, store the jam in the refrigerator and use within 2-3 weeks.