In the present paper we prove that for any open connected set Ω ⊂ Rn+1, n≥ 1 , and any E⊂ ∂Ω with Hn(E) < ∞, absolute continuity of the harmonic measure ω with respect to the Hausdorff measure on E implies that ω| E is rectifiable. This solves an open problem on harmonic measure which turns out to be an old conjecture even in the planar case n= 1. © 2016, Springer International Publishing.