River Syfynwy (Welsh: Afon Syfynwy, Syfnwy or Syfni) is a river entirely within Pembrokeshire, Wales, rising in the Preseli Hills, feeding the Rosebush and Llys y Fran reservoirs and joining the Eastern Cleddau to the south. It is a river considered to be important as an ecological indicator and part is in a site of special scientific interest.