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This study investigates the performance degradation of high-capacity Free-Space Optical (FSO) communication systems, particularly Dense Wavelength Division Multiplexing (DWDM), under Malaysian tropical weather conditions. Conventional FSO systems rely on strict Line-of-Sight (LoS) and are highly vulnerable to atmospheric attenuation, geometric spreading, and physical blockage, resulting in reduced transmission range, unstable signals, and poor reliability. To address these challenges, this study proposes a passive Reconfigurable Intelligent Surface (RIS) as a controllable optical reflector to redirect and reconstruct the propagation path without active amplification. The system is modelled using OptiSystem integrated with a MATLAB-based RIS model in a dual-hop configuration and extended from clear-sky conditions to tropical weather scenarios including rain, humidity, and temperature variations. A 32-channel DWDM system at 1550 nm is implemented, and performance is evaluated using received optical power, SNR, BER, OSNR, constellation, and eye diagram under attenuation ranging from 0.47 to 65 dB/km. Results show that conventional systems suffer severe degradation, with received power dropping to approximately −100 dBm, whereas the RIS-assisted system achieves significant improvements of 13–25 dB, with enhanced signal quality, lower BER, and more stable performance across all conditions. These findings demonstrate that RIS effectively mitigates LoS limitations and enables reliable, high-capacity optical wireless communication in tropical environments, making it a practical solution for next-generation communication systems.