We measured the polarization-transfer coefficient K y y of the fusion reaction 2 H( d, p ) 3 H at E d = 58 keV at the laboratory reaction angle θ = 45 • . The result is compared with theoretical predictions based on Faddeev-Yakubovsky equations, calculated with and without inclusion of the Coulomb interaction, and a prediction based on a T-matrix parametrization of all available data of this reaction at energies below 500 keV.