Transition from laminar to turbulent flow drastically changes the mixing, transport, and drag properties of fluids, yet when and how turbulence emerges is elusive even for simple flow within pipes and rectangular channels 1,2 . Unlike the onset of temporal disorder, which is identified as the universal route to chaos in confined flows 3,4 , characterization of the onset of spatio-temporal disorder has been an outstanding challenge because turbulent domains irregularly decay or spread as they propagate downstream. Here, through extensive experimental investigation of channel flow, we identify a distinctive transition with critical behavior. Turbulent domains continuously injected from an inlet ultimately decayed, or in contrast, spread depending on flow rates. Near a transition point, critical behavior was observed. We investigate both spatial and temporal dynamics of turbulent clusters, measuring four critical exponents, a universal scaling function and a scaling relation, all in agreement with the (2+1)-dimensional directed percolation universality class. Transition to turbulence in open shear flows such as pipe flow and channel flow has been a difficult puzzle for over 130 years 1 .In such flows, the laminar flow becomes turbulent despite its linear stability 5-7 . Also, turbulent structures tend to be localized; laminar states do not break up into turbulent states unless they are invaded by turbulent neighbors. If the tendency for invasion by a turbulent state increases, the turbulent state will eventually spread over the entire space. It is this behavior that led Pomeau to conjecture that the spatio-temporal intermittency observed at the transition from laminar flow to turbulence belongs to the directed percolation (DP) universality class [8][9][10][11] . DP is a stochastic spreading process of an active (turbulent) state with a single absorbing state 12 , which diverse phenomena such as spreading of epidemics, fires, synchronization 13 , and granular flows 14 potentially belong to. Thus, if the transition is continuous and the interaction is shortranged, then universal critical exponents are expected 12,15 . The linear stability of the laminar flow and recent experimental findings of two competing processes (namely decaying and splitting of a turbulent puff) in pipe flow 16 qualitatively support this analogy 17-20 . However, direct characterization of the transition has been lacking. This