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. Avoid pressing down on the raspberry pulp if you want a more clear liquid.
Let the syrup cool down to room temperature, then pour it into a clean glass jar or another airtight container.
Notes
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.