The interaction of [Ru(phen)dppz-idzo] (phen = 1,10-phenanthroline, dppz-idzo = dppz-imidazolone) with triplex RNA poly(U)·poly(A)*poly(U) was carried out by using spectroscopic and viscometric techniques in this work. Luminescent titrations suggest that [Ru(phen)dppz-idzo] shows better selectivity for poly(U)·poly(A)*poly(U) compared with poly(U)·poly(A) and poly(U), this complex exhibits a "light switch" effect with an emission enhancement factor of about 123 in the presence of poly(U)·poly(A)*poly(U). Significantly, this "light switch" behavior could even be observed by the naked eye under irradiation with UV light. To our knowledge, [Ru(bpy)dppz-idzo] is the first small molecule able to serve as a colorimetric molecular "light switch" for the triplex poly(U)·poly(A)*poly(U). Combined with the spectral and viscometric results as well as [Ru(phen)dppz-idzo] stabilizing the template duplex poly(U)·poly(A), we speculate that [Ru(phen)dppz-idzo] prefers to bind with the Hoogsteen base-paired strand (the third strand) of the triplex, thus the intercalating [Ru(phen)dppz-idzo] stabilizing the third strand is more marked in comparison with the Watson-Crick base-paired duplex of the triplex. The results obtained here may be useful for understanding the interaction of triplex RNA poly(U)·poly(A)*poly(U) with small molecule, particularly ruthenium(II) complexes.