The dynamic light scattering (DLS) technique was applied to polystyrene fractions of different molecular weights dissolved in the near-Theta solvent trans-decahydronaphthalene and confined in a porous glass bead. Diffusion coefficients were measured for the polymer in the interior of the porous medium and in the surrounding free solution. Diffusion, when normalized to take into account solvent viscosity and temperature, was slower in the Theta solvent than in the good solvent 2-fluorotoluene, a result being ascribed to a stronger hydrodynamic interaction for a compact Gaussian chain in the pore channels than for a chain with excluded volume. At higher temperatures, the decrease in the reduced diffusivity was larger than expected for the increased radius of gyration of the polystyrene chain in the better solvent. At the same time, the DLS autocorrelation function exhibited a pronounced deviation from a single-exponential decay. Enhanced interaction between swelling polymer chains in the narrow pore channels is considered to have increased the concentration coefficient of the friction coefficient for translational motion and thus to have resulted in the decrease of the diffusion coefficient at a low, but nonzero, polymer concentration.