In this paper we prove a variant of the usual Helmholtz decomposition for an exterior domain $\Omega$. The main difference is that in the decomposition $h=\nabla f+v$ for $h\in L^p(\Omega)$, with $\nabla f,v\in L^p(\Omega)^n$, div$v=0$, we assume that $f$, not $v$, vanishes on the boundary of the domain. Generally speaking, we are also more precise about which function spaces $f$ belongs to. It should be mentioned that our results, though not our proofs, have some strong overlap with some results in [C. G. Simader and H. Sohr, The Dirichlet Problem for the Laplacian in Bounded and Unbounded Domains, Longman, Harlow, UK, 1996], where a somewhat different approach is used to obtain a similar decomposition. We needed the type of decomposition we construct here for our study of the exterior two-dimensional quasi-geostrophic equation (cf. [L. Kosloff and T. Schonbek, Adv. Differential Equations, 17 (2012), pp. 173--200], [L. Kosloff and T. Schonbek, Discrete Contin. Dyn. Syst. Ser. S, 7 (2014), pp. 1025--1043]...