We analyzed the presence of microplastics (<5 mm) in the sediment of the Guarapiranga Reservoir (São Paulo, Brazil) from a drilling core collected in 2010 that covers the whole sedimentary history of the reservoir. The reservoir supplies drinking water to 3.8 million people living in the biggest metropolitan region in South America. We examined sediment samples from layers representing decades starting from the 1920s. The core chronology was determined by 210Pb dating. The polymeric composition was identified using Raman and µ-FTIR spectroscopy. Microplastics were introduced in the reservoir during the 1940s. The concentration progressively increased, ranging from 955 to 15199 parts/kg sediment dw, and from 0.429 to 6.858 mm3/kg sediment dw (both 1940s and 2010s, respectively). Fibers are predominant, usually 2-5 mm in length. Most of the particles are composed of polyethylene terephthalate (PET), polystyrene (PS), and polypropylene (PP). The diversity in the composition of the polymers increased over the decades, being dominated by PET in the last two decades. The watershed is characterized by areas with high urban density (up to 100 inhabitants/km2) and deficient sanitation infrastructure. Therefore, the introduction of microplastics in the reservoir is mainly derived from domestic sewage, runoff, and waste disposal in the margins. The presence of microplastics in the sediment correlates with the growing human activity around the reservoir and coincides with the proposed beginning of the Anthropocene (1940s), which reinforces the use of microplastics as a technofossil marker.