Is psychology an art or a science?

Both.
Psychology is a degree offered by most accredited universities as either an art or as a science. BSc Psychology and BA Psychology are two distinct degrees offered around the same subject matter with fundamentally different approaches. Some elements of psychology, such as Pavlov's experiments, are scientific in nature. Others techniques such as hypnosis and introspection are cognitive rather than scientific. Ultimately, this is why psychology and psychiatry are different practices. A psychologist works explicitly on behaviour and personal improvement through therapy which either provokes thought or uses cues and environmental associations to adjust patient behaviour. A psychiatrist is a medical doctor who has a specialty in mental disorders, which allows them to use the facilities of clinics and hospitals to perform brain scans and issue medication to patients as additional therapeutic aids. In essence, these two kinds of psycho-specialists embody the difference between psychology as an art or science.