An accurate, precise, and simple spectrophotometric method is proposed for the determination of the antithyroid drug carbimazole using ruthenium(III) chloride. This method is based on the formation at room temperature of a stable yellow-colored complex with λ$_{max}$ 370 nm. The method has a Beer's law range of 7.81 to 1.95 $\times $ 10$^{2}$ $\mu $g mL$^{-1}$, molar absorptivity coefficient $\varepsilon =$ 1.519 $\times $ 10$^{3}$ L mol$^{-1}$ cm$^{-1}$, and correlation coefficient of 0.997. The metal:ligand (M:L) ratio of the complex was confirmed by the mole ratio and Job's method of continuous variation, suggesting a 1:2 stoichiometry. The formed carbimazole:Ru(III) complex was confirmed by spectral studies, including Fourier transform infrared, nuclear magnetic resonance, electron spin resonance, magnetic susceptibility, thermal analysis, and powder--X-ray diffraction. Our method could be adopted for routine analysis of carbimazole, due to its ease, affordability, and effectiveness.