We present two-photon photoassociation to the least-bound vibrational level of the X1Σ+g electronic ground state of the 86Sr2 dimer and measure a binding energy of Eb=−83.00(7)(20)kHz. Because of the very small binding energy, this is a halo state corresponding to the scattering resonance for two 86Sr atoms at low temperature. The measured binding energy, combined with universal theory for a very weakly bound state on a potential that asymptotes to a van der Waals form, is used to determine an s-wave scattering length a=810.6(3)(9)a0, which is consistent with, but substantially more accurate than, the previously determined a=798(12)a0 found from mass scaling and precision spectroscopy of other Sr isotopes. For the intermediate state, we use a bound level on the metastable 1S0−3P1 potential. Large sensitivity of the dimer binding energy to light near resonant with the bound-bound transition to the intermediate state suggests that 86Sr has great promise for manipulating atom interactions optically and probing naturally occurring Efimov states.