We report a high-energy and high-gain fiber regenerative amplifier for narrow-bandwidth nanosecond laser pulses that uses a Yb-doped photonic crystal fiber. The input pulse energy is 270 pJ for a 3.5 ns laser pulse with 0.3 nm (FWHM) bandwidth. At a pump laser power of 8.6 W at 974 nm, pulse energies up to 746 μJ with 1.2% (rms) energy stability are generated. To the best of our knowledge, this is the highest energy obtained in a fiber-based regenerative amplifier. A high-energy, nearly diffraction-limited, singlemode beam with a high gain of 64 dB shows promise for future application in the front ends of high-power laser facilities.