Implantable cardiac devices, such as implantable cardioverter-defibrillators (ICDs) and pacemakers, play a crucial role in the management of patients with severe cardiac arrhythmias. These devices enable continuous monitoring and regulation of cardiac activity, thereby preventing life-threatening arrhythmic events. However, their function may be compromised by electrolyte imbalances, among which hyperkalemia is particularly significant. This condition may result in inappropriate shocks or misdetection of arrhythmias. We present the case of a 42-year-old male with a history of chronic heart failure and end-stage renal disease, who was admitted to a cardiology center for cardiovascular assessment and further management. In 2023, the patient underwent implantation of a subcutaneous implantable cardioverter-defibrillator (S-ICD) due to progressive heart failure. During the course of treatment, the patient experienced an inappropriate S-ICD shock triggered by hyperkalemia (serum potassium concentration of 7.21 mmol/L, which led to increased T-wave oversensing and subsequent inappropriate high-energy therapy). Implantable devices are a cornerstone in the treatment of patients with advanced cardiac arrhythmias; however, their effectiveness is highly dependent on the proper monitoring of serum electrolyte levels. Hyperkalemia can interfere with device sensing and therapy delivery, emphasizing the importance of regular electrolyte assessment, particularly potassium, in patients with implanted cardiac devices.