We present a compilation of strontium, carbon, and oxygen isotope compositions of roughly 10,000 marine carbonate rocks of Archean - Ordovician age (3800 Ma -- 450 Ma). The Precambrian Marine Carbonate Isotope Database (PMCID) has been compiled from 152 published and 3 unpublished articles and books of the past 40 years. Also included are 30 categories of relevant metadata that allow detailed comparisons and quality assessments of the isotope data to be made. The PMCID will be updated periodically as new data and better age constraints come to light. Here we outline the structure of the first published version of the database and its inherent merits and limitations.