Thermal expansion of synthetic coesite was studied with synchrotron powder X-ray diffraction in the temperature range of 100–1000 K. We determined the unit cell parameters of monoclinic coesite ( a, b, c , and β ) every 50 K in this temperature range. We observed that a and b parameters increase with increasing temperature, while c decreases. The β angle also decreases with temperature and approaches 120°. As a result, the unit cell volume expands by only 0.7% in this temperature range. Our measurements provide thermal expansion coefficients of coesite as a function of temperature: it increases from 3.4 × 10 −6 K −1 at 100 K to 9.3 × 10 −6 K −1 at 600 K and remains nearly constant above this temperature. The Suzuki model based on the zero-pressure Mie–Grüneisen equation of state was implemented to fit the unit cell volume data. The refined parameters are V 0  = 546.30(2) Å 3 , Q  = 7.20(12) × 10 6 J/mol and θ D  = 1018(43) K, where θ D is the Debye temperature and V 0 is the unit cell volume at 0 K with an assumption that K ′ is equal to 1.8. The obtained Debye temperature is consistent with that determined in a previous study for heat capacity measurements.