Gastrointestinal disorders, frequently associated with microbiome imbalances, represent a widespread health challenge in horses. Biochar has the potential to modulate the gut microbiome. However, its in vivo effects on horses remain unknown. This study aimed to evaluate the effects of dietary biochar supplementation on the faecal bacterial community in horses. We hypothesised that biochar would increase the abundance of beneficial bacterial groups, particularly those involved in fibre degradation, such as Lachnospiraceae and Ruminococcaceae. Eight adult warmblood horses (12.1 +/- 5.3 years, 613.8 +/- 39.3 kg) were used in a crossover study with two 15-day periods. The horses received either a control diet or the same diet supplemented with biochar (1% of diet dry matter). Faecal samples were collected for 16S rRNA gene sequencing. Biochar supplementation did not affect the alpha diversity (Shannon index: Control 8.2, Biochar 8.0; SEM = 0.16; p = 0.298) or beta diversity (p = 0.248; PERMANOVA). At the phylum level, Firmicutes (Control 53.1%, Biochar 51.2%; p = 0.522) and Bacteroidota (Control 30.2%, Biochar 31.6%; p = 0.183) were dominant with no significant changes in their relative abundances. No significant differences were observed in the relative abundance of any major bacterial families, including Lachnospiraceae (Control 13.7%, Biochar 12.9%; p = 0.444) or Ruminococcaceae (Control 2.8%, Biochar 2.5%; p = 0.062). In conclusion, biochar supplementation at 1% of diet dry matter had negligible effects on bacterial diversity and major bacterial taxa in horse faeces, suggesting limited microbiome-modulating potential at the practical inclusion level.