The attached dataset and code accompanies a Data in Brief article, "Monthly nitrogen point-source loading dataset for the Long Island Sound Watershed." Abstract: Quantifying point source nitrogen loads to the Long Island Sound watershed is important because they contribute to the annual occurrence of hypoxia in Long Island Sound. However, the data are not easily accessible and are not available at a central location, making it difficult to characterize the magnitude of loading, seasonal patterns and long-term trends. For the period between April 1989 and September 2021, we gathered all available monthly nitrogen data for wastewater treatment plants within the Long Island Sound watershed from the U.S. Environmental Protection Agency Integrated Compliance Information System-National Pollutant Discharge Elimination System (ICIS-NPDES) and Permit Compliance System, and Connecticut Department of Energy and Environmental Protection databases. Data were checked for quality assurance and unreasonable outliers were imputed with means of other observations within the same year and season. Estimates were also imputed for missing observations. We publish both raw data with missing observations and a curated dataset with imputed estimates for missing observations. Monthly point source nitrogen load data can be used for water quality modeling and to infer, by subtraction, the relative contribution of nonpoint sources to gauged watershed loads. Additionally, the scripts used to reformat data from the ICIS could be repurposed to collect similar data in other regions.