AbstractSound velocities of iron and iron-based alloys at high pressure and high temperature are crucial for understanding the composition and structure of Earth’s and other telluric planetary cores. In this study, we performed ultrasonic interferometric measurements of both compressional (νP) and shear (νS) velocities on a polycrystalline body-centered-cubic (bcc)-Fe90Ni10 up to 8 GPa and 773 K. The elastic moduli and their pressure and temperature derivatives are derived from least-square fits to third-order finite strain equations, yielding KS0 = 154.2(8) GPa, G0 = 73.2(2) GPa, KS0′ = 4.6(2), G0′ = 1.5(1), ∂KS/∂T = –0.028(1) GPa/K, and ∂G/∂T = –0.023(1) GPa/K. A comparison with literature data on bcc-Fe suggests that nickel not only decreases both P and S wave velocities but also weakens the temperature effects on the elastic moduli of Fe-Ni alloys.