The YTH domain family member 3 gene (YTHDF3) encodes a reader of the abundant N6-methyladenosine (m6A) modification of eukaryotic mRNA, which plays an essential role in regulating mRNA stability and is necessary to achieve normal development of the central nervous system in animal models. YTHDF3 has not previously been implicated in Mendelian disease despite a high probability of loss of function intolerance and statistical evidence of enrichment for gene-disruptive de novo variants in large-scale studies of individuals with intellectual disability and/or developmental delay. We report four individuals with deletion of 8q12.3, deletion size 1.38 – 2.60 Mb, encompassing YTHDF3, three of them were de novo, and in one case, the inheritance was unknown. Common features of the individuals (age range, 4 to 22 years) were developmental delay and/or intellectual disability. Two individuals underwent squint surgery. We suggest that haploinsufficiency of YTHDF3 causes a neurodevelopmental disorder with developmental delay and intellectual disability of variable degree.