Lightweight, flexible solar cells from III-V semiconductors offer new application opportunities for devices that require a power supply, such as cars, drones, satellites, or wearable devices, due to their outstanding efficiency and power-to-weight ratio (specific power). However, the specific power and stability of flexible photovoltaic (PV) devices need to be enhanced for use in such applications because current flexible PV devices are vulnerable to moisture and heat. Here, we develop ultra-lightweight, flexible InGaP/GaAs tandem solar cells with a dual-function encapsulation layer that serves as both a moisture barrier and an antireflection coating for the active device layer. Using a thin polymer film as a substrate and an ultrathin metal as a bonding layer, the total weight of the device is dramatically reduced. Therefore, the specific power of our solar cells is remarkably high with a value of over 5000 W/kg under the AM 1.5G solar spectrum. Additionally, there is no degradation even if the solar cells are exposed to harsh environmental conditions.