Ternary Sn-In-Cu alloys are prepared and equilibrated at 250 °C for 2 to 20 weeks. The phases formed in these alloys are experimentally determined. The 250 °C Sn-In-Cu isothermal section is established according to the phase equilibrium information obtained in this study and that of the three constituent binary systems. It has eight single-phase regions, namely liquid, δ1-Cu41Sn11, ε-Cu3Sn, δ2-Cu7In3, η-(Cu6Sn5, Cu2In), Cu11In9, Cu2In3Sn, and α-(Cu) phases, 14 two-phase regions, and seven three-phase regions. In the Sn-In-Cu system at 250 °C, the η-Cu6Sn5 and η-Cu2In phases form a continuous solid solution and the ternary Cu2In3Sn compound is observed. The δ1-Cu41Sn11 phase is stabilized at 250 °C with the introduction of indium although it transforms into α-(Cu) and ε-Cu3Sn phases via a eutectoid reaction around 350 °C in the binary Sn-Cu system. Except for the Cu11In9 phase and the Cu2In3Sn ternary compound, the other binary compounds all have significant indium and tin mutual solubilities.