  The superconducting diode effect -- the dependence of critical current on its
direction -- can arise from the simultaneous breaking of inversion and
time-reversal symmetry in a superconductor and has gained interest for its
potential applications in superconducting electronics. In this letter, we study
the effect in a two-dimensional topological insulator (2D TI) in both a uniform
geometry as well as in a long Josephson junction. We show that in the presence
of Zeeman fields, a circulating edge current enables a large non-reciprocity of
the critical current. We find a maximum diode efficiency 1 for the uniform 2D
TI and $(\sqrt{2} - 1)^2 \approx 0.17$ for the long Josephson junction.
