The adiabatic‐mode solution is widely used for modeling sound propagation in slowly varying ocean environments. However, there are several variations of adiabatic mode solutions and there has been confusion about which form is correct. The standard derivation yields a form that isnotreciprocal. As such, it has often been modified in anadhocfashion to produce a form that is reciprocal. This lack of reciprocity is due to the common assumption that the environment is rotationally symmetric about the source. By assuming instead a plane‐geometry environment a new adiabatic form that is reciprocal is derived.