An injection molding system of plastic material, includes a press having first and a second pressure plates, suitable for closing between them a mold, by applying a closing force, and a distribution and injection assembly suitable for distributing and injecting a fluid plastic material into the mold and in turn comprising an intermediate hot distribution plate interposed between a bottom plate and a nozzle-bearing plate, provided for cooperating into contact and in sliding relationship, along a respective sliding and contact surface with the end of one or more injection nozzles fixed on the nozzle-bearing plate, in order to distribute the fluid plastic material to be injected into the mold, wherein the distribution and injection assembly is arranged adjacent to and along a side of the mold, not associated with the first and the second pressure plates so as not to be subject to the closing force of the mold.