In this Letter, a proof is provided for the reciprocity between modal scattering coefficients of the acoustic waveguides connected by a junction enclosure. The result holds for all waveguide modes and for junction enclosures with locally reactive boundary conditions away from the interfaces between the junction and waveguides. Also provided is a physical interpretation of the reciprocity of the modal scattering coefficients. The scattering of two-dimensional waveguide modes by a right-angled bend in a rectangular duct is used as an illustrating example. © 2013 Acoustical Society of America.