  We propose a new type of supercurrent diode effect on the surface of a
superconductor with surface states under in-plane magnetic fields. Surface
supercurrent diode effect can lead to a perfect supercurrent diode in a
considerably wide range of fields. For comparison, the conventional
supercurrent diode effect due to the spin-orbit coupling in a two-dimensional
superconductor cannot be perfect in usual cases. Candidates such as the (001)
surface of iron-based superconductors BaFe$_{2-x}$Co$_x$As$_2$ are discussed.
Calculations are performed under the Ginzburg-Landau formalism.
