We conducted an experiment on lambs to determine the effect of infection with the gastrointestinal nematode (GIN) Haemonchus contortus on growth performance, nutrient composition, microbiota, and ruminal fermentation in vivo and in vitro. Twelve female lambs were equally divided into two groups: uninfected animals (CON) and animals infected with approximately 5000 third-stage ISE larvae derived from the MHCo1 strain of H. contortus (INF). Two lambs per group were killed on days 48, 49, and 50 after infection and ruminal content was collected separately from each lamb. Batch cultures of ruminal fluid from CON and INF were incubated in vitro with 0.25 g meadow hay, a plant mixture and chicory used as substrates. The ruminal populations of protozoa, bacteria, total Archaea, and the archaean orders Methanobacteriales and Methanomicrobiales were significantly higher in INF than CON. In vivo concentrations of n-butyrate, iso-butyrate, and iso-valerate were significantly lower in INF than CON. In vitro values after 24 h of ruminal fermentation varied within the substrate. GIN infection by H. contortus was associated with increased methanogenesis. In conclusion, chicory was a promising substrate for mitigating methane production and may modulate the ruminal microbiome of GIN-infected lambs due to its chemical and phytochemical composition.