The notion of quantum spin-liquids (QSLs), antiferromagnets with quantum fluctuation-driven disordered ground states, is now firmly established in one-dimensional (1D) spin systems as well as in their ladder cousins. The spin-1/2 organic insulator ฮบ-(bis(ethylenedithio)-tetrathiafulvalene) 2 Cu 2 (CN) 3 (ฮบ-(BEDT-TTF) 2 Cu 2 (CN) 3 ; ref. 1) with a 2D triangular lattice structure is very likely to be the first experimental realization of this exotic state in D โฅ 2. Of crucial importance is to unveil the nature of the low-lying elementary spin excitations 2,3 , particularly the presence/absence of a 'spin gap', which will provide vital information on the universality class of this putative QSL. Here, we report on our thermal-transport measurements carried out down to 80 mK. We find, rather unexpectedly, unambiguous evidence for the absence of a gapless excitation, which sharply contradicts recent reports of heat capacity measurements 4 . The low-energy physics of this intriguing system needs be reinterpreted in light of the present results indicating a spin-gapped QSL phase.In antiferromagnetically coupled spin systems, geometrical frustrations enhance quantum fluctuations. Largely triggered by the proposal of the resonating-valence-bond theory for S = 1/2 degrees of freedom residing on a frustrated twodimensional (2D) triangular lattice and its possible application to high-T c cuprates with a doped 2D square lattice 8,9 , realizing/detecting QSLs in 2D systems has been a longsought goal. Recently, discoveries of QSL states on S = 1/2 triangular lattices have been reported in organic compounds, ฮบ-(BEDT-TTF) 2 Cu 2 (CN) 3 (Fig. , inset) , C 2 H 5 (CH 3 ) 3 Sb