Place strawberries in a large pot and mash with a potato masher. Mix in sugar and lemon juice.
Mix on medium heat until the sugar is dissolved, then bring to a boil. Boil for 25-30 minutes or until the jam reaches 220°F.
Pour into clean jars that have been soaked in hot water. Let the jam cool until it reaches room temperature, then place in the fridge. The jam will store for at least a month.