Pour the pomegranate juice into a saucepan. Add sugar and stir to dissolve. Place the saucepan on the stove over a medium-high heat.
Cook for anywhere between 45-60 minutes, stirring occasionally. You want it to reduce until it coats the back of a ladle and forms a thick syrup (it will continue to thicken as it cools and sits in the fridge so use your best judgement here).
Once it has reduced, set aside to cool completely. Decant into a glass/steel jar and refrigerate. Keeps well refrigerated for up to a month.