We present comprehensive seeing statistics for the San Pedro Mártir site derived from theThirty Meter Telescope site-selection data. The observations were obtained between 2004 and2008 with a Differential Image Motion Monitor (DIMM) and a Multi-Aperture ScintillationSensor (MASS) combined instrument (MASS–DIMM). The parameters that are statisticallyanalysed here are the whole atmosphere seeing (measured by the DIMM), the free atmosphereseeing (measured by the MASS) and the ground-layer (GL) seeing, which is the differencebetween the total seeing and the free atmosphere seeing.We have made a careful data coveragestudy, along with statistical distributions of simultaneousMASS–DIMMseeingmeasurements,in order to investigate the nightly, monthly, seasonal, annual and global behaviour, as wellas possible hourly seeing trends. Although this campaign covers five years, the sampling isuneven, with 2006 and 2007 being the best sampled years in terms of seasonal coverage.The overall results yield a median seeing of 0.78 arcsec (DIMM), 0.37 arcsec (MASS) and0.59 arcsec (GL). Therefore, the strongest contribution to the whole atmosphere seeing comesfrom a strong ground layer. We find that the best season is summer, while the worst season iswinter, in accordance with previous studies. It is worth noting that the best yearly results arecorrelated with the best sampled years. The hourly analysis shows that there is no statisticallysignificant tendency for seeing degradation towards dawn. The seeing values are slightly largerthan those reported previously, which might be the result of climate changes.