The mathematical modeling of glued-laminated (glulam) timber beams is greatly simplified by considering the material to be isotropic in the plane of the cross section. In this paper, the hypothesis of transverse isotropy for southern pine glulam beams is verified through an experimental torsion program. Solid-sawn samples of rectangular cross sections that exhibit rhombic symmetry were tested in torsion about their longitudinal axes (\IL\N) to compute the principal shear moduli in the radial (\IG\dL\dR\N) and tangential (\IG\dL\dT\N) directions. The small difference between \IG\dL\dR\N) and \IG\dL\dT\N was indicative of the transversely isotropic behavior of southern pine beams. Based on this behavior, the shear moduli (\IG\N) of rectangular and circular glulam samples were computed from torsion tests and Saint-Venant’s isotropic torsion solution. The experimental results show that the depth-to-width ratios of the cross sections have no significant influence on the computed shear moduli of the glulam samples; thus, Saint-Venant’s torsion solution can accurately represent the linear torsional response of southern pine glulam beams.