Combine the blackberries, lemon juice, sugar, and water in a small saucepan. Place the saucepan over medium heat. Lower the heat once the first bubbles appear and the mixture starts simmering.

Let the mixture simmer for around 5 minutes, stirring occasionally and mashing the blackberries a bit. Remove it from the heat.

Let the syrup cool to room temperature. Once it has cooled strain it through a muslin cloth or fine-mesh strainer.

Pour the blackberry syrup into a clean glass jar or airtight container.

Leftover syrup can be stored in the refrigerator for up to 2 weeks.