A probabilistically shaped 9-ary carrierless amplitude and phase (CAP-9) optical modulation format based on a reverse distribution model is proposed. We experimentally demonstrate the transmission of 33.3-Gb  /  s PS-CAP-9 signals over 25-km standard single-mode fiber in an intensity-modulation and direct-detection system. The results indicate that the proposed scheme outperforms the conventional uniform CAP-16 and the conventional PS-CAP-16 in receiver sensitivity by 1.2 and 0.4 dB at the threshold of 3.8  ×  10^−  3   BER, respectively.