We describe the topological Hochschild homology of the periodic complex K-theory spectrum, T HH(KU), as a commutative KU-algebra: it is equivalent to KU [K(Z, 3)] and to F (ΣKU Q), where F is the free commutative KU-algebra functor on a KU-module. Moreover, F (ΣKU Q) KU ∨ ΣKU Q , a square-zero extension. In order to prove these results, we first establish that topological Hochschild homology commutes, as an algebra, with localization at an element. Then, we prove that T HH n (KU), the n-fold iteration of T HH(KU), i.e. T n ⊗ KU , is equivalent to KU [G] where G is a certain product of integral Eilenberg-Mac Lane spaces, and to a free commutative KU-algebra on a rational KU-module. We prove that S n ⊗KU is equivalent to KU [K(Z, n + 2)] and to F (Σ n KU Q). We describe the topological André-Quillen homology of KU as KU Q .