The crystal structure of resmetirom heminonahydrate Form CSI has been solved and refined using synchrotron X-ray powder diffraction data and optimized using density functional theory techniques. Form CSI had been described previously as a dihydrate. Resmetirom heminonahydrate Form CSI crystallizes in space group P-1 (#2) with a = 11.3094(23), b = 15.158(6), c = 16.570(7) Å, α = 67.405(13), β = 74.425(7), γ = 69.526(7)°, V = 2,427.2(4) Å3, and Z = 4 at 298 K. The crystal structure consists of layers of resmetirom molecules parallel to the bc-plane. These layers are separated by water-rich layers also parallel to the bc-plane. A strong N–H···O links the two resmetirom molecules. The equivalent amino group in the other molecule acts as a donor to a water molecule. A number of C–H···O and C–H···N hydrogen bonds also contribute to the lattice energy. Water molecules act as donors to both O and N in the resmetirom molecules. The structure is more complicated than a hydrogen-bonded framework of resmetirom molecules with water in the pores. The powder pattern has been submitted to the International Centre for Diffraction Data (ICDD) for inclusion in the Powder Diffraction File™ (PDF®).