This dataset consists of surface measurements of partial pressure of CO2 (pCO2), pH on total scale, water temperature, salinity, and other variables in the Casco Bay, Gulf of Maine from 2015-04-23 to 2020-06-12. The central goal of this project was to install continuous monitoring sensors in Casco Bay to measure ocean and coastal acidification parameters at high temporal resolution. The system was installed at the Southern Maine Community College (SMCC) pier in South Portland, Maine. The pier is located in the Portland Channel, an important southern outlet of Casco Bay, and near outlets of the Fore and Presumpscot rivers in a relatively urban area of Casco Bay. This location was selected because it is nearshore, accessible, and has historic nutrient data collected by the Friends of Casco Bay. Sensors for temperature/conductivity/salinity, dissolved oxygen, pH and pCO2 were mounted to a ‘lander’ frame, which was lowered via davit to the bottom. Sensor data were thus collected approximately 1m from the bottom, and at varying depth depending on the tide.