Bilirubin is predominantly formed in the liver as a result of breakdown of hemoglobin. Knowing the concentration of bilirubin in serum is important in evaluating the health of the liver, and for the diagnosis of hyperbilirubinemia (a condition that afflicts approximately 60% of full-term and 80% of pre-term newborns). This paper describes the design and fabrication of a potentiometric sensor for the determination of free bilirubin in serum. The sensor has a polymeric ion-selective membrane, and selectively measures free ionic bilirubin (“unbound” bilirubin – i.e., bilirubin not complexed to albumin or other complexing agents), in the presence of other anions — chloride, phosphate, pyruvate, deoxycholate, and lactate — also present in serum. The linear response range of the sensor (1.0 mM to 0.10 μM bilirubin, measured in a sodium phosphate buffer with pH 8.6) covers the clinically-relevant concentration of bilirubin in serum (5–500 μM). Free bilirubin could be detected in human blood serum with this potentiometric sensor. The components of the potentiometric bilirubin sensor were embedded in a paper-based device to provide a sensor that is disposable and easy to use, and thus is suitable for applications at the point-of-care. The paper-based potentiometric bilirubin sensor exhibited a response range of 5.0–0.10 mM (sufficient to cover the clinically-relevant concentration of bilirubin in serum). Only 15 μL of sample is required for measurement of the concentration of free bilirubin, and the analysis can be performed in less than two minutes.