The present technology relates to a solid-state image sensor, an imaging device, and an electronic device capable of switching FD conversion efficiency in all pixels of a solid-state image sensor. A photodiode performs photoelectric conversion on incident light. A floating diffusion (FD) stores charge obtained by the photodiode. FD2, which is a second FD to which the capacity of an additional capacitor MIM is added, adds the capacity to the FD. The additional capacitor MIM is constituted by a first electrode formed by a wiring layer and a second electrode formed by a metallic light blocking film provided on a surface of a substrate on which the photodiode is formed. Switching between the FD and FD+FD2 allows switching of the FD conversion efficiency. The present technology is applicable to a CMOS image sensor.