A professional who provides therapy, such as a psychotherapist, physical therapist, or occupational therapist. The specific type is usually clear from context.
彼女はセラピストとして病院で働いている。
She works as a therapist at a hospital.
ストレスがたまったので、セラピストに相談した。
I was stressed, so I consulted a therapist.
She's been seeing a therapist because of stress.
She got certified as an aromatherapist.