To address the growing water stress in the Gabe`s region of Tunisia, experiments were conducted on a conventional pyramid solar still and finned configurations with 3, 6, 9, 12, and 15 PCM-filled square fins. This study presents, for the first time, the integration of square-section fins filled with phase change material (PCM) in a pyramid solar still (PSS), a configuration not previously explored in Tunisia or internationally. Unlike prior PCMfin studies that focused on hemispherical or double-slope stills using cylindrical or flat fins, the square-fin geometry increases the effective heat-transfer surface and ensures more uniform PCM melting and solidification. The study also develops a thermal optimization strategy tailored to the unique heat-transfer behavior of pyramid stills, characterized by strong vertical natural convection and non-uniform temperature fields. Results show that the 12-fin configuration achieved the highest performance, with energy and exergy efficiencies of 36.5 % and 3.37 %, representing improvements of 82.5 % and 206 % over the conventional system. It also achieved the lowest cost per liter at $0.04, the shortest energy payback time, and a net COQ mitigation of 12.96 tons. These findings demonstrate that PCM-filled square fins enhance thermal storage and sustain productivity during evening hours. This study provides a novel, practical, and regionally relevant solution for freshwater scarcity in arid regions such as Gabe`s and offers insights for optimizing solar desalination systems globally. From a sustainable development perspective, the proposed system simultaneously addresses freshwater scarcity, cost-effectiveness, and environmental impact, confirming its suitability for sustainable freshwater production in arid and semi-arid regions.