Psychology is a very broad field, it can range from hardcore biology ( Biopsych & Neuroscience, evolutionary psychology, animal behavior/ behavioral ecology) to applied biology and applied social science ( Psychopathology, cognitive science, developmental psychology) and then to purely social science ( Social Psychology). It fundamentally is a social science because its main interest is to dissect behavior and cognition, but it of all the social science is likely the most truly scientific and similar to biology.
I personally am both a psychology major and a neurophysiology major and I can say that they draw from each other a lot.