“God is a circle, whose centre is everywhere, whose circumference is nowhere.” Jung repeats this ancient saying often when describing the Self. This article frames the saying as a type of koan and uses Zen Buddhism's conception of the Buddha-nature to unlock the psychological meaning of Jung's saying and to suggest new perspectives on Jung's notion of the Self. Parallels between Jung's notion of the Self and Zen's Buddha-nature emerge with particular clarity in Mysterium Coniunctionis in connection with Jung's discussion of the alchemical unus mundus and caelum. The essay further delves into the relationship between Zen teachings on enlightenment and Jung's notion of individuation and explores how these insights can inform Jungian theory and practice. By addressing the implications of Zen concepts for understanding the Self, the essay ultimately seeks to illuminate the shared spiritual dimensions of Jungian psychology and Zen Buddhism and to provide new perspectives on both.