Electromagnetically induced transparency (EIT) is an important quantum optical phenomenon which provides a crucial tool for light manipulation. However, typically the transparency window is broad, limited by the coherence time of the metastable state. Here we show that extremely narrow transparency window can be realized using nuclear spin induced transparency (NSIT), which is achieved by combining optical field, magnetic field and the spin-exchange interaction between noble-gas nuclear spins and alkali-metal electronic spins. The width of the NSIT window can be several orders of magnitude smaller than that of conventional EIT, and even reaches sub-mHz range due to the long coherence time of nuclear spins. The scheme holds great potential for applications in slow light and magnetic field sensing.