We present measurements of the branching fraction B and longitudinal polarization fraction f_L for B^0 → a_1(1260)^(+)a_1(1260)^(-) decays, with a_1(1260)^(±) → π^(-)π^(+)π^(±). The data sample, collected with the BABAR detector at the SLAC National Accelerator Laboratory, represents 465 × 10^6 produced BB(overbar) pairs. We measure B(B^0 → a_1(1260)^(+)a_1(1260)^-) × [B(a_1(1260)^+ → π^(-)π^(+)π^(+))]^2 = (11.8±2.6±1.6) ×10^(-6) and f_L = 0.31±0.22±0.10, where the first uncertainty is statistical and the second systematic. The decay mode is measured with a significance of 5.0 standard deviations including systematic uncertainties
