An expansion tank for a cooling system of a motor vehicle has an inlet mouth and is provided with a shell in plastic material having a wall which defines a cavity for containing a coolant liquid; the shell has, in addition, a projecting collar, at the inlet mouth, and a first inner tubular wall, which protrudes downwards into the cavity and defines a passage with a lower outlet opening, to make the coolant liquid flow into the cavity; the shell has, in addition, a second tubular wall, projecting upwards into the cavity and defining a compartment which is vertically aligned with the passage and has an upper overflow opening vertically arranged at a height that is equal to or greater than that of the outlet opening.