The spin diffusion coefficientD of a polarized two-dimensional degenerate Fermi fluid is evaluated as a model of submonolayer3He adsorbed on a flat substrate. The problem is formulated to account fully for Fermi liquid and spin polarization effects. Because in two dimensions, for zero or weak polarization, the only allowed scattering angles are zero and pgr, the transport coefficients become expressible in terms of Landau parameters without approximation. The dependence ofD on polarizationP is given for arbitraryP. For unpolarized and weakly polarized systems, in particular, it is shown thatD –1 simT 2lnT andT 2lnP, respectively. The two-dimensional kinetic equation is found to be exactly solvable.