Abstract We report results of the first reverberation mapping campaign of I Zwicky 1 during 2014–2016, which showed unambiguous reverberations of the broad Hβ line emission in the varying optical continuum. From analysis using several methods, we obtain a reverberation lag of . Taking a virial factor of , we find a black hole mass of from the mean spectra. The accretion rate is estimated to be , suggesting a super-Eddington accretor, where L _Edd is the Eddington luminosity and c is the speed of light. By decomposing Hubble Space Telescope images, we find that the stellar mass of the bulge of its host galaxy is . This leads to a black hole to bulge mass ratio of ∼10⁻⁴, which is significantly smaller than that of classical bulges and elliptical galaxies. After subtracting the host contamination from the observed luminosity, we find that I Zw 1 follows the empirical relation.