This paper presents a new property called the Duality Theorem for Continuous Hartley Transform (CHT). Hartley Transform is an integral transform related to the renowned Fourier Transform but has remained in the oblivion from mathematicians and engineers alike. Many of the properties of the Hartley Transform are akin to those of the Fourier Transform subject to minor differences. A formal derivation of the Duality Theorem corresponding to Continuous Hartley Transform is given which was hitherto not mentioned or derived in the literature. The usage of Duality Theorem helps in finding the time-domain function from the Hartley frequency domain and vice versa thereby reducing considerable labour involved in integration and computation time.