This article presents an investigation of the phase equilibria in the Sn-Bi-In ternary alloy system, performed both by theoretical and experimental methods. Following the regular solution model and a standard thermochemical calculation, a theoretical evaluation of the phase equilibrium in the entire ternary system is conducted. The thermodynamic parameters required for the calculation are initially obtained by fitting the model to existing data available from prior studies. The theoretical results are then validated and further improved by experimental work in which alloys with several critical compositions were chosen and examined. In the experimental work, differential scanning calorimetry (DSC), X-ray diffraction (XRD), and energy-dispersive X-ray (EDX) spectroscopy were jointly used to identify the transition temperatures and the phases in the microstructure. The resulting phase diagram agrees well both with the existing data and with the data from the current experiments. However, different from previous findings, this study finds a nonbinary nature of the Sn-BiIn and Sn-BiIn 2 quasi-binaries and nine invariant reactions, one eutectic, six peritectic and two peritectoid. The phase-reaction scheme (Scheil diagram), the liquidus projection, and the phase diagram, covering entire compositional ranges, are established.