This paper presents a 5 Gb/s radiation-tolerant fully-differential optical receiver designed and implemented in a commercial 0.13 μm CMOS process. When connected to a PIN-diode, the optical receiver displays a sensitivity better than -19 dBm for a BER of 10-12. The differential output across an external 100 Ω load remains constant at 400 mVpp even for signals near the sensitivity limit. The chip achieves an overall transimpedance gain of 20 kΩ at the worst case. The total power consumption of the chip is less than 120 mW and the chip die size is 0.75 mm × 1.25 mm. A novel biasing circuit for the PIN-diode is designed to maintain sufficient voltage across it in the presence of large DC leakage currents induced by ionizing radiation. Irradiation testing of the chip shows basically no performance degradation after a total ionization dose of 2 MGy.