Put water, lemon juice, sugar, and lemon zest in a medium saucepan and bring to a boil.
Once boiling, lower heat and let mixture simmer for about 10 minutes, until syrup has thickened (note that it will continue to thicken as it cools).
Strain syrup into a bowl, leaving the zest behind.
Keep syrup in bowl or pour into a mason jar and place in fridge to cool completely before using.