Let d be a given positive integer and let {R j } d j=1 denote the collection of Riesz transforms on R d . For 1 < p < ∞, we determine the best constant C p such that the following holds. For any locally integrable function f on R d and any j ∈ {1, 2, . . . , d},A related statement for Riesz transforms on spheres is also established. The proofs exploit Gundy-Varopoulos representation of Riesz transforms and appropriate inequality for orthogonal martingales.