In a medium saucepan, combine rhubarb, sugar, and water.
Bring to a gentle boil over medium heat, then reduce to a simmer. Cook for 15–20 minutes, stirring occasionally, until the rhubarb breaks down completely.
Stir in lemon juice, if using.
Remove from heat and let cool for about 20 minutes.
Strain through a fine mesh sieve into a clean jar or bowl, pressing down gently to extract the liquid (don’t force solids through).
Discard or save the pulp for smoothies or oatmeal.
Store syrup in an airtight container in the fridge for up to 2 weeks.