<p>Access to freshwater remains a critical global challenge, particularly in arid and off-grid regions where conventional desalination technologies are energy-intensive. Solar stills offer a sustainable alternative; however, the effect of still geometry on thermal behavior and freshwater productivity remains insufficiently understood. Existing studies focus mainly on single geometries or hybrid configurations, leaving a clear knowledge gap in the direct comparative evaluation of different pyramid architectures under identical operating conditions. This study addresses this gap by conducting a combined mathematical and experimental comparison of two newly fabricated solar stills, a Pentagonal Pyramid Solar Still (PPS) and a Square Pyramid Solar Still (SPS), each constructed with an identical basin area of 0.25 m². Experiments were performed under outdoor tropical climatic conditions (solar intensity 600–950 W/m²), and hourly data for temperature, humidity, and distillate yield were recorded. A Python-based transient heat and mass transfer model was developed and validated using statistical indicators (R² = 0.96, MAPE ≈ 3–5%). The findings show that PPS consistently outperformed SPS, producing 18.3% higher daily distillate output, 15.1% greater thermal efficiency, and 17.6% higher exergy efficiency. The performance enhancement is primarily attributed to the additional inclined glass face in the pentagonal geometry, which increases optical exposure, internal radiation trapping, and the effective condensation surface. Economic analysis further demonstrates that PPS reduces the cost per liter of freshwater by nearly 12% compared with SPS. Overall, the study establishes that geometric optimization plays a decisive role in solar desalination performance, and the validated mathematical model provides a practical framework for future solar still design and scaling in decentralized water-scarce regions.</p>