Monitoring cell viability and proliferation in real-time provides a more comprehensive picture of the changes cells undergo during their lifecycle than can be achieved using traditional end-point assays. Particularly for drug screening applications, high-temporal resolution cell viability data could inform decisions on drug application protocols that might lead to better treatment outcomes. We describe a CMOS biosensor that monitors cell viability through high-resolution capacitance measurements of cell adhesion quality. The system consists of a 3 × 3 mm2 chip with an array of 16 sensors, on-chip digitization, and serial data output that can be interfaced with inexpensive off-the-shelf components. An imaging system was developed to provide ground-truth data of cell coverage concurrently with data recordings. Results showed the sensor's ability to detect single-cell binding events, track cell morphology changes, and monitor cell motility. A chemotherapeutic assay was conducted to examine dose-dependent cytotoxic effects on drug-resistant and drug-sensitive cancer cell lines. Concentrations higher than 5 μM elicited cytotoxic effects on both cell lines, while a dose of 1 μM allowed discrimination of the two cell types. The system demonstrates the use of real-time capacitance measurements as a proof-of-concept tool that has potential to hasten the drug development process.