Where the two concepts come from
Jung introduced the collective unconscious formally in Die Struktur der Seele (1927) and developed it through the 1930s–40s. By the 1954 Archetypes of the Collective Unconscious (CW 9.i) the theory was mature: a psychic layer beneath the personal unconscious, containing inherited archetypal forms (the Great Mother, the Wise Old Man, the Shadow, the Anima, etc.), shared across humanity because we share a nervous system and an evolutionary history. Buddha-nature (佛性) is older and more foundational. The doctrine appears in the Mahāparinirvāṇa Sūtra (5th century), the Laṅkāvatāra Sūtra, and the tathāgatagarbha literature. The Chán Platform Sūtra (6th century) centers it: the Sixth Patriarch Huìnéng argues that every being already possesses Buddha-nature, and realization is not attaining something new but recognizing what was never absent.
The 70% overlap: where they genuinely say the same thing
Both frameworks claim (a) there is a universal substrate to mind, (b) ordinary ego-consciousness is cut off from it, (c) a specific practice — in Jung, active imagination and dream work; in Zen, zazen and kōan — can re-establish contact, and (d) the transformation this produces is not "adding" a new psychological skill but re-grounding the subject in a deeper layer of themselves. This 70% overlap explains why the late-20th-century conversation between Jungians and Zen teachers (Philip Kapleau, Taizan Maezumi, Shoji Muramoto) was so productive. They were, at a coarse grain, describing the same territory.
The 30% divergence: content vs. emptiness
The real difference is whether the substrate has structure. Jung's collective unconscious is FULL. It is populated with archetypes — specific structural patterns that appear in dreams, myths, and clinical material across cultures. The work of analysis is to recognize, integrate, and negotiate with these archetypes. Buddha-nature, as read in Mādhyamaka-derived Zen, is EMPTY (śūnya). Not "empty of everything" in a nihilistic sense, but empty of inherent self-nature. The Heart Sūtra's "form is emptiness, emptiness is form" is the canonical formulation. When Huìnéng asks Huáiràng "what is it that thus comes?" — and Huáiràng eventually answers "to say it is any thing misses" — this is precisely the refusal of Jungian-style content. This difference matters clinically. A Jungian analysand who reports a vision of the Great Mother has found something real and negotiable. A Zen practitioner who reports a vision of Avalokiteśvara is told by any competent teacher to "let it go" — makyō, "demon realm," not the goal.
How to integrate the two in practice
The practical integration — which is what matters if you're doing this work — is sequential. Jungian work is especially powerful on the way in: surfacing shadow material, recognizing archetypal patterns in your relationships, integrating the Anima/Animus. You cannot skip this layer by going straight to zazen; unprocessed shadow will leak into and contaminate meditation practice (this is documented repeatedly in Aitken, Kapleau, and Beck). Zen work becomes primary on the way through: once the archetypal layer has been sufficiently engaged, the practice shifts to recognizing that the whole architecture — personal unconscious, collective unconscious, archetypes — is itself dependently arisen, not a final ground. This is what Dōgen means in Genjōkōan (1233): "To study the Buddha Way is to study the self. To study the self is to forget the self." The forgetting requires the self to first be present. Ordering matters. Reverse it and you get either spiritual bypassing (Zen first) or endless archetypal fascination (Jung only).