Background and Objective <p><i>Nigella sativa</i> is a widely used medicinal plant with several potential therapeutic uses. This study aimed to investigate the possible beneficial cardioprotective effect of <i>Nigella sativa</i> in pediatric patients with type 1 diabetes mellitus.</p> Methods <p>Sixty children and adolescents with type 1 diabetes were randomized into two groups: group I (<i>n</i> = 30) who received <i>Nigella sativa</i> seed oil 450 mg twice daily after meals for 3 months in addition to insulin, and group II (<i>n</i> = 30) who received insulin alone. Echocardiographic examinations were performed before and after the treatment. The lipid profile, malondialdehyde, nitric oxide, tumor necrosis factor-α, transforming growth factor-β, and troponin I were also measured before and after <i>Nigella sativa</i> treatment.</p> Results <p>After 3 months of <i>Nigella sativa</i> administration, group I had significantly lower cholesterol and low-density lipoprotein-cholesterol, malondialdehyde, nitric oxide, tumor necrosis factor-α, transforming growth factor-β, and troponin I levels compared with their pretreatment levels and compared with group II. In addition, group I had a significantly higher left ventricular <i>E</i>′/<i>A</i>′ ratio and two-dimensional left ventricular global longitudinal strain&#xa0;(2D-LV GLS) compared with baseline values and compared with group II after treatment.</p> Conclusions <p><i>Nigella sativa</i> can improve subclinical left ventricular dysfunction in pediatric patients with type 1 diabetes&#xa0;mellitus.</p> <p><b>Clinical Trial Registration:</b> this clinical trial was registered at <a href="http://www.pactr.org">www.pactr.org</a> with ID: PACTR202302478939306.</p>