Background <p>Metabolic bariatric surgery (MBS) candidates undergo a comprehensive nutritional preparation process by a registered dietitian (RD). The effect of eHealth interventions on the MBS preparation process is unknown.</p> Objectives <p>To assess the impact of adding an application to the nutritional preparation process on pre-surgery nutritional knowledge, physical, and behavioral parameters among MBS candidates.</p> Methods <p>An open-label randomized controlled trial among MBS candidates. All participants received 3–6 meetings with an RD and the intervention group also received access to an application containing information modules and a communication platform. Data was collected at baseline and end of preparation.</p> Results <p>Forty participants were recruited, of them 67.5% women, with a mean age and body mass index of 34 ± 10.1&#xa0;years and 43.5 ± 6.0&#xa0;kg/m², respectively. Nutritional knowledge, anthropometrics, functionality, adherence to most behavioral recommendations, and subjective state of health improved in both groups (<i>P</i> Time ≤ 0.044). Physical activity initiation (i.e., beginning of regular exercise engagement) was higher among the intervention group (40% at baseline and 68% at end of preparation vs 35% at baseline and 32% at end of preparation for interventions and controls, respectively, <i>P</i> Time × Group = 0.026). The application was rated as providing added value (8.2 on a scale of 1 (no added value) to 10 (meaningful added value)).</p> Conclusions <p>Nutrition preparation process with an RD improved MBS knowledge, adherence to behavioral recommendations, subjective state of health, and modestly enhanced weight and functionality outcomes among MBS candidates. Although rated as having an added value, incorporating an application had only a minimal impact on these outcomes.</p>