Calcium is considered important in buffering excess stomach acid in mammals, including horses. Control of stomach acid is important in preventing the development of ulcers within the stomach lining, which, in horses, are considered to be caused by acid splashing. Algae supplements contain various minerals which are in natural form, as seen in all plant and feedstuffs. The current trial was conducted to examine if a high calcium algae supplement had any impact on gastric ulceration in horses, which may be due to buffering stomach acid, reducing the pH in a gradual manner, without resorting to medication. Ten horses, of either thoroughbred, standardbred or sport horse breed, were selected on the basis of the presence of ulcers in their stomach, as ascertained by endoscopy. The average ulceration score before algae supplementation was 2.2 ± 0.75 according to the EGUC scoring system. The horses were then maintained on their normal diet (unchanged from the initial ulcer scoring) by the owner with the addition of 40 g per day of the high calcium, algae based Maxia Complete® (Seahorse Supplements Ltd, Christchurch, NZ) for thirty days (T30). All horses were then re endoscoped to assess any change in ulceration score. All horses showed a significant improvement in ulcer score, with seven having a score of zero (fully healed, no evidence of further ulceration) and two with a score of one (some residual inflammation or keratinosis in areas of healed ulcers). This resulted in a mean score of 0.3 ± 0.48 (P < 0.0001: T0 versus T30) at the end of the study. This trial demonstrated that feeding an organic form of high calcium from algae reduced ulceration in horses.