Monosomy 21 is an exceedingly rare and fatal chromosomal anomaly. Mosaic monosomy 21, however, can be observed in living patients. There have been discussions on whether there are liveborn cases with true mosaic full monosomy 21. Here, we report the case of a 13‐year‐old patient with mosaic full monosomy 21 who presented with postnatal microcephaly, low weight, facial dysmorphisms, developmental delay, and severe intellectual disability. To the best of our knowledge, this is the oldest patient with mosaic full monosomy 21 described so far and the first reported in Brazil.