Multiferroics have attracted research interest for decades due to their potential applications. Multiferroics are relatively scarce due to the mutual exclusivity between magnetic and ferroelectric properties. New room-temperature multiferroics are looking forward to be exploited. Room-temperature multiferroic properties have been observed in a new rare-earth iron garnet (REIG), Nd3Fe5O12 (NdIG). The dielectric properties of NdIG over a wide temperature range (300 K-650 K) at different frequencies (100 Hz-1 MHz) and in the temperature range from 70 K to 300 K at 100 Hz are investigated herein. The results show that this material has two dielectric relaxations in the temperature range from 290 K to 525 K and a dielectric anomaly around the magnetic ordering temperature (565 K). The first dielectric relaxation in the temperature range from 290 K to 500 K is attributed to the conduction with an activation energy of 0.75 eV. The other dielectric relaxation in an elevated temperature range has an activation energy of 0.87 eV, which originates from the inhomogeneous structure. Furthermore, the ferroelectric polarization of approximately 28 μC/m2 at 77 K is determined by the positive-up-negative-down method and induced by the effective magnetic field created by the iron sublattice magnetic ordering and electric-dipole moments of the neodymium ions.