Objective: To investigate the predictive value of nutritional status in heart failure (HF) patients with an implantable cardioverter defibrillator (ICD), and to identify factors associated with ICD discharge and mortality. Methods: This retrospective study was conducted by analyzing data from 2017 to 2021. HF patients who underwent ICD implantation for primary prevention were included. Follow-up visits were continued until December 2022. Patients were examined based on ICD shock occurrence (ICD-A: appropriate shock), ICD non-discharge (ICD-X), and mortality. Nutritional status was assessed by the Prognostic Nutritional Index (PNI) and the Controlling Nutritional Status (CONUT) scores. Results: A total of 221 patients were included in the study, 86 of whom were in the ICD-A group (135 in the ICD-X group). Age and sex distribution were similar in these groups. The all-cause mortality rate was 20.36%. A PNI with a cut-off value of <47.25 and a CONUT score with a cut-off value of >2.5 were able to significantly predict all-cause mortality. The PNI had a greater area under the curve compared to the CONUT. Non-ischemic cardiomyopathy and high left-ventricle end-systolic diameter (ESD) were independently associated with appropriate ICD shock. Low systolic blood pressure, high ESD, low sodium, low total cholesterol, low (<47.25) PNI, and ICD shock were independently associated with all-cause mortality. Conclusions: Malnutrition appears to be associated with mortality in patients with primary-prevention ICDs, and the PNI appears to be a more useful indicator than the CONUT for determining the risk of mortality in these patients.