© 2022 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.The aim of this study is to determine the effectiveness of dried tomato pulp, as a soluble dietary fibre source, using [0 (control group), 2.5, 5.0, 10.0, and 20.0% (experimental groups) as dry matter-DM] in horse total mixed ration (TMR) on the in vitro fermentation parameters. The in vitro total gas production (TGP), methane emission, gas yield (GY24), metabolic energy (ME), in vitro true dry matter disappearance (T-DMd), short-chain fatty acids (SCFA) molarities, partial factor (PF24), microbial crude protein production (MCP), and pH value at 24 h of fermentation in horse TMR’s were determined. In the study, in vitro TGP, GY24, PF24, ME, MCP, SCFA, and methane emission of horse TMR’s were not changed by 2.5 and 5.0% tomato pulp supplementation (P > 0.05). The in vitro TGP (73.6 vs. 85.6 and 105.9 mL/g DM), GY24 (189.4 vs. 210.8 and 286.3 mL for g T-DMd), PF24 (5.9 vs. 5.2 and 4.2 mg/mL for GY24) ME (5.3 vs. 5.6 and 6.1 MJ/kg DM), MCP (270.1 vs. 258.2 and 215.1 mg/g DM), SCFA (0.32 vs. 0.37 and 0.46 mmol/0.2 g), and methane emission (0.28 vs. 0.33 and 0.48 ml/g DM) of horse TMR’s positively affected by 10 and 20% tomato pulp supplementation (P < 0.05). The in vitro T-DMd values of TMRs and pH value of in vitro digestion fluid were similar for both control and experimental groups (P > 0.05). As a result, it was demonstrated that tomato pulp, which uses as dietary fibre and CP source, did not affect negatively the digestion values of the working horse diet. However, it has been considered that tomato pulp using at 10 and 20% ratios increased the in vitro digestion values of the working horse diet. Determining the in vitro digestion levels of tomato pulp, which is a source of soluble dietary fibre, for horses will guide future in vitro and in vivo studies.