Equine gastric disease (EGD) is a common condition in performance horses (Equus caballus), potentially compromising behaviour, performance, and welfare. EGD is often attributed to high-starch, high-sugar feeds and limited forage. Evidence for diet-induced changes on digestive microbiota is lacking. Nine elite showjumping horses were housed at the same performance yard with standardised diet and management throughout the study. Horses were transitioned from a high-sugar and -starch (31%) feed to a low-starch and -sugar (16.5%) concentrate feed. Gastroscopies, blood, and faecal samples were taken pre- and 12 weeks post-diet change. Squamous and glandular ulceration was blindly graded a posteriori using 0–4 scores and faecal microbiota profiled using 16S rRNA gene amplicon sequencing. Total (t_(1,8) = −6.17, p < 0.001; Pre: 4 [0–5], Post: 1 [0–2]), squamous (t_(1,8) = −5.32, p < 0.001; Pre: 1 [0–3], Post: 0 [0–1]), and glandular (t_(1,8) = −2.53, p = 0.04; Pre: 2.5 [0–4], Post: 0 [0–2]) disease improved following the introduction of a low-starch diet. Diet change did not impact microbiota communities (PERMANOVA: F_(1,16) = 1.37, p = 0.15, r² = 0.08), but Firmicute to Bacteroidota (F/B) ratio reduced (t_(1,8) = −3.13, p = 0.01; Pre: 2.07 ± 0.21 vs. Post: 1.29 ± 0.14). Lower F/B ratios were associated with reduced total EGD scores (ChiSq_(1,17) = 3.83, p = 0.05). Low-starch diets did not influence faecal microbiota diversity but aided gastric disease healing and reduced F/B ratios in elite showjumpers during a competition season without medication.