This paper presents the study of subcutaneous solar energy harvesting for implantable sensor systems. The characteristics of a flexible solar panel under a 3 mm thick porcine skin are measured under different ambient light conditions. The output power of the solar panel when covered by the skin varies from tens of micro Watts to a few milli Watts depending on the light source. A low-power implantable sensor prototype is proposed to evaluate the performance of the subcutaneous solar energy harvester. It consists of a power management circuit, a temperature sensor and a Bluetooth low energy (BLE) module. The average working current of the prototype is $400 \mu \mathrm {A}$ (transient BLE transmission current is 8 mA), while its sleep current is only $7 \mu \mathrm {A}$. Experimental results show that the subcutaneous solar energy harvester illuminated by both sunlight and artificial light sources can power the implantable prototype.