A redox, substitution and self-assembly reaction offers a novel dysprosium selenoarsenate(III) with As³⁺ [Dy₂(tepa)₂(μ₂-OH)₂Cl₂]₃[As₃Se₆]₂ (1, tepa = tetraethylenepentamine), which provides the first example of chair conformation ring [As₃Se₆]³⁻ combined with lanthanide complexes as counterions. 1 exhibits a photocurrent response and slow magnetic relaxation behavior.