Physical layer security (PLS) is an attractive paradigm for protecting information confidentiality without the use of traditional key-based cryptography. This paper proposes a design of probabilistic constellation shaping to improve the PLS performance of optical wireless channels using M-ary pulse amplitude modulation (M-PAM) with the signal-dependent noise model. Specifically, the proposed design aims to maximize the reliability (i.e., minimization of the bit error rate - BER) of the legitimate user's channel while guaranteeing that the modulationconstrained secrecy capacity is at least equal to that achieved by the uniform signaling. For this purpose, a novel upper bound for the BER of M-PAM constellation with signal-dependent noise channels with arbitrary input distribution is derived. Simulation results reveal that the proposed PCS design achieves a considerably lower BER of the legitimate user's channel while attaining the same secrecy capacity as the uniform signaling. © 2025 IEEE.