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Myotonic dystrophy type 1 (DM1) is a progressive, multisystemic disorder in which gastrointestinal (GI) involvement is prevalent but frequently underestimated in routine clinical care. This study aimed to evaluate the GI manifestations and symptom burden in patients with adult-onset DM1. We performed a retrospective study in patients with genetically confirmed adult-onset DM1. GI symptom burden was assessed through three sources: (1) patient-reported outcome measures (PROMs), specifically the Gastrointestinal Symptom Rating Scale (GSRS) and the Sydney Swallow Questionnaire (SSQ), (2) an open-ended question asking patients to name their five most bothersome symptoms, and (3) GI symptoms and diagnoses documented in their medical records. Associations between GI symptoms and sex, age, disease duration, Body Mass Index (BMI), and CTG repeat size were analysed. Ninety-five patients with adult-onset DM1 were included (57% female; mean age 44.2±11.8 years). GI symptoms imposed a substantial burden, as 54% of patients exceeded the GSRS clinical threshold and 53% exceeded the SSQ threshold. In open-ended reporting, 31% of patients named a gastrointestinal problem among their most bothersome symptoms, ranking it fourth among all complaints, 13% named swallowing difficulties. In line with the PROM findings, medical record review demonstrated a high prevalence of GI manifestations, including dysphagia (59%), diarrhoea (35%), and constipation (33%). Female sex was strongly associated with higher odds of constipation (OR 9.25,p<0.01), diarrhoea (OR 3.45,p=0.02), and abdominal pain (OR 7.40,p<0.01). Increasing age and longer disease duration were associated with higher odds of constipation, dysphagia, cholelithiasis, and non-alcoholic liver steatosis (p<0.05). No associations were observed for CTG repeat size or BMI. GI symptoms are highly prevalent and burdensome in adult-onset DM1. Female sex, longer disease duration, and older age were associated with a higher symptom prevalence. Structured screening using PROMs should be integrated into multidisciplinary care to ensure that these symptoms are detected promptly and addressed adequately.