Abstract Background Nutritional status is an important prognostic factor in Amyotrophic Lateral Sclerosis (ALS). We wished to study the safety, tolerability and efficacy of nutritional counseling with or without an mHealth application to maintain or increase body weight in ALS, compared to standard care. Methods In this randomized open-label, standard-of-care-controlled, single-center clinical trial, we randomly assigned adults with ALS to one of three nutritional interventions: counseling by their physician or nurse (“standard care”), counseling by a registered dietitian (RD) (“in-person”), or counseling supported by a mHealth app (“mHealth”). Both intervention arms received tailored nutrition recommendations and recorded dietary intake and weight biweekly (mHealth) or monthly (in-person). The primary outcome of weight and secondary and tertiary outcomes of calorie intake, ALSFRS-R, and quality of life (QOL) were recorded at each clinic visit and analyzed in an ITT mixed model analysis. Results A total of 88 participants were enrolled of whom 78 were included in this analysis. The three arms were well-balanced except for excess males in the mHealth arm and greater weight lost at baseline in the in-person arm. Participants in the mHealth arm increased their calorie intake at month 3 to an average of 94% (95% CI: 85, 103) of recommended calories, compared to 81% (95% CI: 72, 91, p = 0.06 vs. mHealth) in the standard care arm. After 6 months, calorie intake was not different among the three arms. Overall weight was stable across all three groups. QOL scores in the mHealth arm were stable over 3 months (0.3 points, 95% CI: − 1.7, 2.2) compared to worsening in standard care (− 2.1 points, 95% CI: − 4.0, − 0.2, p = 0.09 vs. mHealth), but all scores declined by 6 months. ALSFRS-R total scores declined by an average of − 2.6 points (95% CI: − 5.1, − 0.1) over 6 months in the mHealth arm (p = 0.13 vs. standard care) compared to − 5.8 points (95% CI: − 8.2, − 3.4, p = 0.74 vs. standard care) in the in-person and − 5.2 points (95% CI: − 7.6, − 2.9) in the standard care arm. Conclusions Nutritional counseling by a registered dietitian (with or without support by an mHealth app) is safe but did not maintain weight significantly better than standard care in ALS patients. Trial registration https://clinicaltrials.gov/ identifier NCT02418546. Registered April 16, 2015.