1-Methylthymine is of particular interest because it can be considered as a simple model of thymidine, in which deoxyribose attaches to thymine precisely at the N1 atom. The structure of this molecule is still unknown and so far has been experimentally studied for the first time in this work. The semiexperimental equilibrium structural parameters (r(e)(se)/<(se)(e)) of 1-methylthymine have been determined by the gas electron diffraction (GED) method, taking into account vibrational corrections calculated with the use of the anharmonic (cubic) MP2/cc-pVTZ force constants. The methyl torsion around the C-N bond has been treated as a large-amplitude motion. For the first time, the structure of this molecule has been optimized by the very time-consuming coupled-cluster method (CCSD(T)(ae)) with the triple-zeta (cc-pwCVTZ) basis set. The obtained results have been extrapolated to the quadruple-zeta basis set at the MP2 level. It has been revealed that the methylation of uracil, especially at the nitrogen atom, leads to an increase in the flexibility of the nucleobase as well as a noticeable deformation of the pyrimidine ring.