Variable pressure humidification technology is proposed for seawater desalination through evaporation at pressure(s) less than atmospheric and subsequent condensation. Two sub-atmospheric humidification processes (process-I and II) utilizing solar heat are proposed and assessed theoretically. The desalinated water production rate, gained output ratio and an economical comparison between processes are made subject to similar operating conditions. It is deduced that, under similar conditions, pressure reduction from 0.9 bars to 0.1 bars leads to approximately 50% increase in the desalinated water production rate in each of the proposed processes. For humidifier pressures less than atmospheric pressure, the maximum gained output ratio for process-II is higher by 139.13% than that of process-I while the distilled water production rate is lower by 5.71%. Unlike other variable pressure desalination processes, there is no need to use compressor when the vacuum pump is present. To have a comprehensive study of the proposed systems and determine their feasibilities, a cost analysis is conducted. The result of cost analysis indicates that the total cost per liter of distilled water in two proposed process are 0.034 and 0.041 US $/4., respectively. This value is estimated to be reduced to 0.002 US $/L for an industrial scale variable pressure humidification-dehumidification plant.