The southwestern region of the Dominican Republic (Enriquillo Valley) contains exceptionally well-preserved, relict marine and saline lake deposits of mid-Holocne age. Abundant euryhaline ostracodes found in this deposit include Cyprideis salebrosa, C. mexicana, C. similis, and C. edentata. Morphometric and geochemical analyses performed on Cyprideis spp. provide high δ18O and δ13C values that are coincident with relative abundances of irregularly shaped pores that permeate the ostracode carapace. We recognize three stratigraphic intervals with distinct ostracode pore shape and stable isotope trends: (I) a 4.5–5.0m interval that contains ostracodes with highly irregular shaped pores (multiradiate) and high amplitude variability in δ18O and δ13C values; (II) a 5.0–5.6m interval comprised of ostracodes with circular pores and an overall trend towards low δ18O and δ13C values; and (III) a 5.6–6.5m interval containing ostracodes with an upward increasing abundance of circular pore shapes coincident with decreasing δ18O and δ13C values. When the Enriquillo lagoon was first separated from the Caribbean Sea approximately 4.3 ka, an arid and evaporative climate led to hypersaline water in a restricted lagoon environment. By the middle to late-Holocene, increased precipitation in the valley resulted in a coastal lake system that became progressively oligohaline. Moderate to small amplitude variability in the salinity proxy data (δ18O) suggest short-term oscillations in the precipitation-evaporation budgets at that time. At least two marine incursions likely contributed to the observed variability in ostracode δ18O and δ13C values. Evidence for abrupt changes in base level indicate that climatic factors or also tectonic activity may have contributed to the observed paleoenvironmental trends recorded in these deposits.