The ocean’s sea surface height (SSH) field is a complex mix of motions in geostrophic balance and high-frequency tides, internal tides and internal gravity waves. The transition scale Lt, at which unbalanced ageostrophic motions dominate balanced geostrophic motions, is documented for the first-time using satellite altimetry. Lt is critical to define the scales where surface geostrophic currents can be inferred from SSH. We use a statistical approach based on the analysis of SSH wavenumber spectra to obtain Lt. Small values of Lt are observed in regions of energetic mesoscale activity, while large values are observed in tropical latitudes and regions of low mesoscale activity. Seasonally, larger Lt values are observed during summer than during winter, following the seasonality of upper ocean dynamics. These results are consistent with recent analyses of in situ observations and high-resolution models. Limitations of our results and implications for nadir and swath altimetric missions are discussed.