Fowl adenoviruses cause three economically important diseases in broiler chicken flocks: hepatitis-hydropericardium syndrome (HHS), inclusion body hepatitis (IBH), and adenoviral gizzard erosion. IBH has not been considered a serious threat in northeast Iran since the last decade, because no major effect on flock performance has been noticed along with a low mortality rate. During this period, all the sporadic IBH outbreaks have also been investigated for HHS without finding any confirmed case. In March 2021, a 15-day-old commercial broiler flock in northeast Iran showed a 50% mortality rate, and birds underwent postmortem examination, histopathology, molecular testing, and phylogenetic analysis for possible disease agents. Typical gross lesions of HHS were observed postmortem that included hydropericardium with an unusual accumulation of jelly-like and straw-colored fluid in the pericardial sac (without right ventricular failure); petechial or ecchymotic hemorrhages on the myocardium, myocardial valves, and endocardium; and discolored and mottled liver along with small white foci and petechial or ecchymotic hemorrhages. Histopathologic analysis showed necrosis of hepatocytes and basophilic inclusion bodies in the livers. The molecular tests performed for detection of fowl adenovirus (FAdV), H5 avian influenza virus, Newcastle disease virus, avian infectious bronchitis virus (IBV), H9N2, chicken infectious anemia virus (CIAV), infectious bursal disease (IBD) virus, Marek's disease virus, Ornithobacterium rhinotracheale, Mycoplasma gallisepticum, and Mycoplasma synoviae turned out positive for FAdV, CIAV, IBD vaccine virus, and IBV serotypes 793B and variant I. The phylogenetic tree based on the hexon gene loop 1 demonstrated a FAdV serotype 4 (FAdV-4) that was identical to Pakistani isolate PARC-1/98. Because it was the first detection of a FAdV-4 in Iran, the stamping out program was applied immediately on the basis of HHS gross lesions and positive PCR reaction on pericardial jelly-like fluid. It seems that this eradication strategy was successful because no outbreaks were noticed for 2 mo after the initial outbreak. It was concluded that the use of gross pathologic baselines, quick diagnosis of disease, and close collaboration between governmental and private sectors were the critical factors that helped locally control the first occurrence of HHS in Iran.