A Horton-Rogers-Lapwood problem for onset of thermal convection in a horizontal porous layer with anisotropic permeability and diffusivity is solved analytically. There is full 3D anisotropy, with the restriction that one of each set of principal axes points in the vertical direction. The porous layer has infinite horizontal extent. The upper and lower boundaries are taken to be impermeable and kept at constant temperatures. The critical Rayleigh number for the onset is calculated as a function of the five parameters governing the anisotropy: two permeability ratios, two diffusivity ratios and the angle (in the horizontal plane) between the principal axes of permeability and diffusivity.