The regionally proximal relation of order d along arithmetic progressions, namely AP [d] for d ∈ N, is introduced and investigated. It turns out that if (X, T ) is a topological dynamical system with AP [d] = ∆, then each ergodic measure of (X, T ) is isomorphic to a d-step pro-nilsystem, and thus (X, T ) has zero entropy. Moreover, it is shown that if (X, T ) is a strictly ergodic distal system with the property that the maximal topological and measurable d-step pro-nilsystems are isomorphic, then AP [d] = RP [d] for each d ∈ N. It follows that for a minimal ∞-pro-nilsystem, AP [d] = RP [d] for each d ∈ N. An example which is a strictly ergodic distal system with discrete spectrum whose maximal equicontinuous factor is not isomorphic to the Kronecker factor is constructed.