Spinoza’s philosophy shows how relationality and, in particular, the human capacity for language provides a model of human personhood in which individual subjectivity and identity exist only through mutually affective relations with the world and with others. Spinoza’s rationalism is sometimes understood as denying the reality of individual persons and dissolving subjectivity in the impersonality of pure being and the immanence of a thoroughly natural cosmos. It is possible, however, to interpret Spinoza differently; indeed much recent work on Spinoza has come from interdisciplinary studies that recognize the importance of his thought for domains as diverse as global politics and economics, literary theory and neuroscience. What these various interdisciplinary studies share is an emphasis on the relational character of Spinoza’s metaphysics. This essential relationality not only offers a model of individual personhood; it offers at the same time a model of how persons are constituted within and contribute to collective scientific, cultural, evolutionary and religious processes. The “geometrical method,” or order of presentation in Spinoza’s text, provides a model of thinking that combines formal and logical relationships with a unique interplay of meaning and program. This complex discursive structure both represents and performs how communicative relationality is able to take place across disparate domains of experience and knowledge. Two examples demonstrate the potential relevance of the idea and practice of relational personhood in Spinoza’s Ethics to contemporary, interdisciplinary concerns. The first is drawn from evolutionary biology; the second draws upon Spinoza’s concept of relational identity to rethink the possible connections between modern philosophical and religious traditions. Spinoza’s Ethics offers definite ways to conceive and to implement interdisciplinary possibilities, particularly by applying the relational conception of personal identity more generally to the identities of collectivities and traditions. In this way, the concept of relational personhood opens out onto a more general framework for rethinking the constructive relationality of groups, traditions, disciplines and ways of life.