Agammaglobulinemia is a primary immunodeficiency disorder characterized by profoundly low or absent serum antibodies and low or absent circulating B cells. The most common form is X-linked agammaglobulinemia (XLA) caused by mutations in BTK gene. The remaining cases, clinically similar to XLA, are autosomal recessive agammaglobulinemia (ARA). Nearly 30% of ARA cases present mutations in the μ heavy constant region gene IGHM. Here, we present a 7-month-old patient, born from non-consanguineous parents, who is affected by ARA due to defect in the μ heavy chain. The genetic study showed that the patient is compound heterozygous for an IGHM gene deletion and the novel nonsense mutation X57331.1:g.275C > A (p.Tyr43*) (ClinVar Accession Number: SCV000537868.1). This finding allows for an adequate genetic counseling to the family and also broadens the spectrum of already described point mutations at this locus. The IGHM gene is very complex and it is likely that yet unidentified mutations appear in other patients.