A UV reactor irradiates a flow of fluid with UV radiation. The reactor comprises: a fluid conduit defined by a heat conducting conduit body comprising one or more heat conducting walls for permitting a flow of fluid therethrough; a UV-LED operatively connected to a PCB and oriented for directing radiation into the fluid conduit. The PCB comprises a heat conducting substrate having a first surface. The conduit body is in thermal contact with the first surface of the heat conducting substrate. Heat is dissipated from the UV-LED via the heat conducting substrate, the thermal contact between the first surface of the heat conducting substrate and the heat conducting conduit body, and from the one or more heat conducting walls of the heat conducting conduit body to the fluid flowing through the fluid conduit.