We construct a five-parameter family of gauge-nonequivalent SU(2) instantons on a noncommutative four sphere and of topological charge equal to 1. These instantons are critical points of a gauge functional and satisfy self-duality equations with respect to a Hodge star operator on forms. They are obtained by acting with a twisted conformal symmetry on a basic instanton canonically associated with a noncommutative instanton bundle on the sphere. A completeness argument for this family is obtained by means of index theorems. The dimension of the ``tangent space'' to the moduli space is computed as the index of a twisted Dirac operator and turns out to be equal to five.