The California State Water Resources Control Board (State Water Board) initiated the Oil and Gas Regional Monitoring Program (RMP) to assess effects of oil and gas development on groundwater designated for any beneficial use. The U.S. Geological Survey (USGS) is the technical lead in conducting the RMP through the California Oil, Gas, and Groundwater (COGG) Program, working in cooperation with the State Water Board, and in partnership with other State and local agencies. The USGS collected and analyzed groundwater and associated quality control (QC) samples in the Santa Maria Valley Oil Field study area during July 2018 – March 2019. Groundwater samples were collected from twenty-one irrigation, one domestic, and three industrial wells located within a five-kilometer buffer zone overlying and surrounding the Santa Maria Valley Oil Field administrative boundary in Santa Barbara County, California. Samples were collected using established groundwater data-collection protocols and procedures. Samples were analyzed for water-quality indicators, major and minor ions, nutrients, trace elements, volatile organic compounds (VOCs), naturally occurring radioactive material, geochemical and age-dating tracers, dissolved organic carbon (DOC), dissolved standard and hydrocarbon gases (methane through hexane), and dissolved noble and atmospheric gases. Two hundred and twenty constituents and water-quality indicators were measured. QC samples collected as part of the Santa Maria Valley Oil Field study included replicates, source-solution blanks, field blanks, laboratory spikes, and split samples for inter-laboratory comparisons.