  We give a gauge-covariant formulation of seven-dimensional super-Yang-Mills
theory in terms of N=1 superfields. Furthermore, we show that five and seven
dimensions are the only cases where such a formulation exists by analysing the
interplay of Lorentz and R symmetries. The action is expressed in terms of
field strengths and a Chern--Simons-like superpotential. Each term is
manifestly N=1 supersymmetric, Lorentz invariant in four dimensions and gauge
invariant under superfield gauge transformations, including those that do not
preserve Wess--Zumino gauge.
