Deficiency of adenosine deaminase 2 (DADA2) is a rare monogenic disease caused by mutations in the adenosine deaminase 2 gene (ADA2). It is characterized by a wide range of manifestations, including systemic inflammation and vasculopathy, early onset stroke (ischemic or hemorrhagic), immunodeficiency, and bone marrow failure. The diagnosis of DADA2 is confirmed by pathogenic mutations in ADA2 or low ADA2 enzymatic activity in the patient. In this study, we present a case of a 24-year-old Saudi male who was admitted with symptomatic anemia, lightheadedness, exertional symptoms, and a history of fever (38.1 C) for one week. Laboratory tests revealed normocytic normochromic anemia, leucopenia, lymphopenia, and neutropenia-autoimmune profile: low C3 and positive anti-ds DNA. The genetic testing revealed two Pathogenic variants, which were identified in ADA2. The diagnosis of DADA2 was made, and the patient received subcutaneous adalimumab 40 mg every two weeks. At the follow-up after one month, he showed improvement in fever, rash, and C-reactive protein (CRP) from (6 to 0.65). In conclusion, we present one of the first cases in Saudi Arabia of an adult patient diagnosed with DADA2 with a unique gene mutation. Adult-onset patients with DADA2 usually have a vague presentation and a relatively narrower phenotype range of symptoms which produce additional challenges for the physician to add DADA2 to the list of differentials. We suggest further studies investigate the genotype-phenotype association, possible clinical presentation, and the development of curative treatments for those cases.