The exponential decay rate of a Timoshenko beam system with boundary damping is studied. By asymptotically analyzing the characteristic determinant of the system, we prove that the Timoshenko beam system is a Riesz system; hence, its decay rate is determined via its spectrum. As a consequence, by showing that the imaginary axis neither has an eigenvalue on it nor is an asymptote of the spectrum, we conclude that the system is exponentially stable.