Interstitial 3p21.31 deletions have been very rarely reported. We describe a 7-year-old boy with global developmental delay, specific facial characteristics, hydronephrosis, and hypothyreosis with a de novo deletion of 3p21.31, encompassing 29 OMIM genes. Despite the wide use of microarrays, no similar case has been reported in the literature so far. Five overlapping cases are deposited in the DECIPHER database, 2 of which have significant overlapping chromosomal aberrations. They both share some phenotypic characteristics with our case, e.g. developmental delay, intellectual disability and facial dysmorphism (arched eyebrows, hypertelorism, low-set ears, and a large nose tip). In addition, loss-of-function mutations in the SETD2 gene (OMIM 612778) of the deleted region have been described in 3 patients, presenting with some similar clinical features, namely overgrowth, intellectual disability, speech delay, hypotonia, autism, and epilepsy. Therefore, SETD2 may explain part of the phenotype in our case. We focused on 3 other genes in the deleted region, based on their known functions, namely CSPG5 (OMIM 606775), PTH1R (OMIM 168468) and SMARCC1 (OMIM 601732), and assessed their potentially important role in describing the patient's phenotype. Additional cases with haploinsufficiency of this region are needed to elucidate further genotype-phenotype correlations.