We report results of the first reverberation mapping campaign of I Zwicky 1 during 2014-2016, which showed unambiguous reverberations of the broad H{beta} line emission in the varying optical continuum. From analysis using several methods, we obtain a reverberation lag of {tau}H{beta}=37.2_-4.9_^+4.5^days. Taking a virial factor of f_BLR_=1, we find a black hole mass of M_{bullet}=9.30-1.38_^+1.26^x10^6^M_{sun} from the mean spectra. The accretion rate is estimated to be 203.9-65.8_^+61.0^L_Edd_c^-2^, suggesting a super-Eddington accretor, where LEdd 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 log(M_bulge_/M_{sun})=10.92+/-0.07. 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 R_BLR{propto}L_5100_^1/2^ relation.