The temperature dependence of the uniaxial surface anisotropy,Ku(T), in ultrathin Fe films (4–10 atomic layers) grown on Cu(100) is determined by comparing the dipole and anisotropy energies at the spin reorientation temperature for films of varying thickness. It is observed that the uniaxial anisotropy has a weak temperature dependence compared to the bulk constants for Fe. The measured exponent of Γu=2.6(0.5) agrees well with thel(l+1)/2 law, which is obtained from a spin fluctuation model wherel=2 at the surface. This shows that the spin reorientation transition can be understood as being driven by thermal spin fluctuations.