Biopsychosocial theory is strengthening psychotherapy by focusing on interactions that engage the attachment system. Art therapy has the potential to align coherently with current theory provided it addresses the value of art in engaging with the attachment system. This article describes an interpersonal theory of art therapy based on a model of ostensive communication that supports the attachment system as a mechanism of change in the art therapy triangular relationship of client, therapist, and art object. A clinical case vignette, derived from a client's description of her art therapy experiences in an audio-image interview, illustrates ostensive communication theory and the development of epistemic trust in art therapy.