Experimental evidence is presented for feedback‐narrowed far‐infrared response of a point contact Josephson junction which is strongly coupled to a resonant cavity. The observed response shows high sensitivity(NEP≲ 10−14W/Hz)and frequency selectivity(Q ≳ 103)and so is of value as a tunable narrow‐band detector of millimeter and submillimeter wave radiation. The Werthamer‐Shapiro theory of the nonlinear coupling of junction current to a resonant cavity has been extended to include the effect of applied radiation at the cavity resonance frequency and is shown to account for the feedback narrowing.