The accelerated datafication of social life has led to increased interest in advancing the use of big data in psychology. However, prevailing methodological preoccupations in big data psychology elide foundational issues which amplify the problems of modernity/coloniality, including the suppression of marginalized psychologies and the widened gap between researchers and their subjects. We propose decolonial foundations for utilizing big data in psychological scholarship. We move beyond prevailing technical concerns with data processing, to engage situated practices of knowledge production. The ontological object of computing shifts from a universal psychology of numerous individuals, to a localized psychology of collectives. Epistemological priorities shift from context-free definition and control, to prioritizing temporal responsiveness and linguistic fidelity to emergent psychological phenomena. Ethical guidelines extend beyond issues of personal privacy, and address sociopolitical concerns in fragile democracies. Finally, reflexivities from the margins account for unequal systems that situate psychologists, big data, and the phenomena they study within the plurality of Global Souths. Two Philippine case studies operationalize these principles through localized data collection practices, participatory algorithmic design, interrogation of bias, and wrestling with state encroachments. We conclude with decolonial directions for advancing equitable practices of knowledge production within the computational and psychological sciences.