Inappropriate management of municipal solid waste dumpsites is a major cause of groundwater contamination in developing countries, but the extent of the problem is not known. This study investigated groundwater quality in the vicinity of Olusosun dumpsite in Lagos, Nigeria, the most populous city in sub-Saharan Africa. During 2020, monthly groundwater samples were collected in 17 wells and boreholes used as drinking water sources, and analysed for 20 physico-chemical parameters. Differences between sites and seasons were statistically assessed, together with changes in water quality index (WQI). The results indicated that heavy metals (Pb2+, Ni+, Mn2+, Fe2+, Cr6+), cations (Ca2+, Mg2+, K+), total hardness and pH were the main parameters impairing water quality. Drinking water quality standards from both the World Health Organization and Nigeria government were exceeded more often in the wet season than in the dry season. Some groundwater properties were negatively correlated with distance to dumpsite (e.g., Fe2+, Pb2+, NO3−). Significant differences between sites were identified, but with no clear spatial trend. WQI varied from excellent (6%–24% of the sites over the study period) to unsuitable for drinking water purposes (12%–18%), with good quality prevailing at most sites (35%–47%). Although groundwater quality declined at 24% of the sites over 2020, the results indicated improvements compared with previous decades. Remediation strategies must be implemented to safeguard public health and the sustainability of water resources.