Pressure–volume–temperature relations have been measured to 33 GPa and 1673 K for natural siderite (Fe0.95Mn0.05CO3) using synchrotron X-ray diffraction with a multianvil apparatus at the ‘SPring-8’ facility. A least-squares fit of the room-temperature compression data to a third-order Birch–Murnaghan equation of state (EOS) yielded K0 = 120 ± 1 GPa and K′ = 3.57 ± 0.09, with fixed V0 = 293.4 ± 0.1 Å3. Further analysis of the high-temperature compression data yielded the temperature derivative of the bulk modulus (∂∂KT/∂∂T)P = −0.015 ± 0.001 GPa/K and zero-pressure thermal expansion α = a0 + a1T with a0 = 3.57(9) × 10−5 K−1 and a1 = 0.06(14) × 10−8 K−2. The analysis of axial compressibility indicates that the c-axis is more compressible (KTc = 59 ± 1 GPa at K′ = 2.7 ± 0.1) than the a-axis (KTa = 166 ± 2 GPa at K′ = 14.0 ± 0.8). The present thermal EOS enables accurate calculation the density and thermodynamic properties of siderite to the deep mantle conditions.