The present technology relates a solid-state imaging element, an electronic apparatus, and a semiconductor device each of which enables deterioration of electrical characteristics in a well region of a semiconductor element formed in a thinned semiconductor substrate to be restrained. A solid-state imaging element as a first aspect of the present technology is a solid-state imaging element constituted by laminating semiconductor substrates in three or more layers, in which of the laminated semiconductor substrates, at least one sheet of the semiconductor substrate is thinned, and an impurity region whose carrier type is the same as that of the thinned semiconductor substrate is formed between a well region and a thinned surface portion in the thinned semiconductor substrate. The present technology can, for example, be applied to a CMOS image sensor.