Subclavian vein aneurysm is a rare form of vascular pathology with poorly understood causes and mechanisms. There is little information in the literature on its prevalence, and it is mainly limited to individual case reports. The article presents a rare case of saccular aneurysm of the subclavian vein in a patient with chronic heart failure and pulmonary hypertension. It was shown that high venous pressure was among the most likely etiological factors for subclavian venous aneurysm. The primary diagnostic method for evaluating the pathology of the principal veins, including aneurysms, is ultrasound duplex scanning. However, the ultrasound examination in the presented case posed several diagnostic challenges due to both the aneurysm’s peculiar location and the comorbidities, necessitating the use of angiographic techniques to differentiate it from arterial aneurysms and other neck masses. Even though venous aneurysms generally are oligosymptomatic with a benign course, in some cases, they can be complicated by thrombosis and thromboembolism. Therefore, the management of venous aneurysms can be surgical or non-surgical. The choice of surgical approach is based on the risk of thromboembolic complications and local symptoms such as pain or cosmetic defect. In the presented case, surgery was not performed due to the high surgical risk associated with decompensated comorbidity.