Add fresh rosemary, a cinnamon stick, sugar, and water to a small pot over medium heat. Stir occasionally until the sugar has completely dissolved. When the liquid comes to a low simmer, cover the pot, and remove the pan from the heat.
Let steep for about 3 minutes, longer if you want a more intense flavor, up to 10 minutes. Strain out the rosemary and cinnamon, reserving the liquid. It should make ¾ cup of syrup.
Transfer it to an airtight container. The syrup keeps in the fridge for about 2 weeks. This syrup is great for teas, homemade sodas, and cocktails.