We show that two important quantities from two disparate areas of complexity theory -Strassen's exponent of matrix multiplication ω and Grothendieck's constant KG -are intimately related. They are different measures of size for the same underlying object -the matrix multiplication tensor, i.e., the 3-tensor or bilinear operator µ l,m,n : F l×m × F m×n → F l×n , (A, B) → AB defined by matrix-matrix product over F = R or C. It is well-known that Strassen's exponent of matrix multiplication is the greatest lower bound on (the log of) a tensor rank of µ l,m,n . We will show that Grothendieck's constant is the least upper bound on a tensor norm of µ l,m,n , taken over all l, m, n ∈ N. Aside from relating the two celebrated quantities, this insight allows us to rewrite Grothendieck's inequality as a norm inequalityKG.We prove that Grothendieck's inequality is unique: If we generalize the (1, 2, ∞)-norm to arbitrary p, q, r ∈ [1, ∞],then (p, q, r) = (1, 2, ∞) is, up to cyclic permutations, the only choice for which µ l,m,n p,q,r is uniformly bounded by a constant independent of l, m, n.