INTRODUCTION: Weight loss in people with amyotrophic lateral sclerosis (pwALS) is a poor prognostic factor. We aimed to develop an intervention to support pwALS to increase calorie intake and explore feasibility and acceptability. METHODS: Intervention development was underpinned by the Capability, Opportunity and Motivation (COM-B) model and Person Based Approach (PBA). Phase 1: healthcare professional (HCP) focus groups (n = 47), national survey (n = 281), mapping review and National Health Service (NHS) organization Freedom of Information requests (n = 251) charted current nutritional support. Systematic reviews of correlates (65 studies) and interventions supporting nutritional behaviors (14 studies), and interviews/focus groups with pwALS (n = 18), carers (n = 18) and HCPs (n = 51) identified barriers/facilitators. Phase 2: Think Aloud interviews (verbalizing thoughts whilst using the intervention) with pwALS (n = 12), carers (n = 10) and HCPs (n = 10). Phase 3: three pilot cycles, interviews with pwALS (n = 9), carers (n = 6) and HCPs (n = 5). RESULTS: Limited evidence-based nutritional guidelines, late dietetic referral post-diagnosis, little HCP training, and few effective interventions were identified. Key facilitators/barriers included capabilities (physical ability), opportunities (social support), and motivations (dietary beliefs). The intervention was developed and piloted to refine content, presentation, and functionality. Concerns around high calorie diets and increasing intake were addressed. The final intervention comprises: (1) interventionist training, (2) calorie target setting, (3) food diaries, (4) feedback provision, (5) online resources, and (6) oral nutritional supplements. User feedback indicates high usability, acceptability, and feasibility. DISCUSSION: The theoretically grounded intervention targets calorie intake through tailored behavior change techniques to support dieticians in practice to deliver personalized care and oral nutritional support for pwALS.