It is shown that an oblique solitary wave may be caused by an oblique strip of enlarged bottom roughness. The analysis is valid for very large Reynolds numbers and very small slopes. Considered are strips with a constant width in the longitudinal direction and with constant roughness. For a strip whose length is much larger than its width, the equations of motion can be simplified by assuming a three-dimensional flow that is independent of the coordinate in the longitudinal direction of the strip. Furthermore, it is assumed that the Froude number in terms of the velocity component normal to the strip is slightly larger than the critical value 1. This facilitates application of an asymptotic expansion method developed previously for plane flow. The analysis does not require modeling of the Reynolds stresses. It turns out that the velocity profile of the three-dimensional flow is not skewed in the first order. The shape, amplitude, and position of the oblique stationary wave are obtained from solutions of the steady-state version of an extended Korteweg-de-Vries equation. In general, the oblique wave is a classical solitary wave with a long, but shallow tail. However, tails are missing for certain combinations of Froude number and parameters characterizing the strip. Measurements and numerical solutions of the full Reynolds equations are already available for orthogonal solitary waves, lending support to the results of the asymptotic analysis. In addition, a novel boundary condition at the free surface is given, and the velocity distribution in fully developed flow is determined.