Phase equilibria in the Mg–Zn–Sr ternary system, in the composition range 0–35 at% Sr at 300 °C, were investigated using two diffusion couples and 23 alloy samples. Scanning electron microscopy equipped with an energy-dispersive spectroscope and electron probe micro-analysis were employed to analyze the constituted phases and their compositions in diffusion couples and alloy samples. X-ray diffraction (XRD) was used to analyze the crystal structure of the constituted phases in selected samples. Four new ternary compounds, Mg11−y Zn y Sr (4.7 < y < 9), Mg44Zn21Sr35, Mg20Zn62Sr18, and Mg55Zn43Sr2 were discovered in the Mg–Zn–Sr ternary system at 300 °C. In total, sixteen three-phase equilibria regions were determined in the present work. The isothermal phase diagram of the Mg–Zn–Sr ternary system, in the composition range 0–35 at% Sr, was constructed at 300 °C.