A resin composition for forming an optical waveguide brings together excellent bending resistance, a low refractive index, and low tackiness suitable for a roll-to-roll (R-to-R) process as a material for forming an optical waveguide, in particular, a material for forming a clad layer. The resin composition for forming an optical waveguide to be used in formation of an optical waveguide includes a polyvinyl acetal compound having a structural unit represented by the following general formula (1) as a main component: in the formula (1), R represents an alkyl group having 1 to 3 carbon atoms, and k, m, and n represent ratios of respective repeating units in a main chain and each represent an integer of 1 or more.