The study aimed to analyze desalination and water purification using modified double-slope passive solar still (MDSPSS). The MDSPSS technique promotes sustainability and minimizes energy consumption, chemical additives, and environmental impacts as compared to other conventional desalination processes. The performance of MDSPSS was evaluated experimentally and theoretically in terms of physicochemical parameters of water quality and productivity. An economic analysis was performed to assess the viability of the process. The experiments were conducted on typical days of June and December in 2017 to account for summer as well as winter conditions. The cumulative distillate productivity was about 8.21 L/day and 6.01 L/day at 1.0 cm water depth during the summer and winter seasons, respectively. The experimental results were compared with standard (water quality) values recommended by the U.S. Environmental Protection Agency and World Health Organization. The cost of the distilled water, as per the economic analysis over 15 years of use, is $0.009 and $0.027 per liter per kWh, respectively, with a currency conversion rate of ₹73.25 for every $, with a payback period of 99 days. The maximum (instantaneous) energy efficiency and overall energy efficiency of MDSPSS in this study are 26.18% and 10.12%, respectively.