The first direct observation of the parametrically generated half‐harmonic voltage in a Josephson tunnel junction is reported. A microwave signal atf=17.25 GHz is applied to the junction dc current biased at zero voltage such that the Josephson plasma resonancefp=f/2. Under these conditions a large‐amplitude microwave signal is emitted atfpprovided the input power exceeds a threshold value. The results are compared to existing theory.