In a small saucepan over medium heat, combine the raspberries, sugar, water, and lemon juice. Bring the mixture to a simmer, stirring as the sugar dissolves.
Once the mixture starts simmering, lower the heat right down. Let it simmer for 10 minutes, stirring occasionally.
Balance a fine mesh strainer over the top of a bowl and tip in the raspberry mixture. Let it drain completely.
Let the syrup cool down to room temperature, then pour it into a clean glass jar or another airtight container.
Fresh fruit syrup like this can be stored in the refrigerator in a sealed container for up to 2 weeks.
For long-term storage, the syrup can be frozen for up to 3 months.
Notes
Fresh or frozen raspberries both work perfectly. No need to thaw frozen berries first.
For longer storage, pour the syrup into ice cube trays and freeze for up to 3 months.
Avoid brown sugar, as it will change the flavor and color of the syrup.