The massive arrival of pelagic sargassum on the Gulf of Mexico coast has become an environmental and socioeconomic challenge, generating high management costs and affecting tourism, fisheries, and coastal ecosystems. In this context, its valorization through anaerobic digestion represents a sustainable alternative for renewable energy production. This study assessed its valorization through anaerobic digestion as a renewable energy route. Pelagic sargassum (Sargassum natans/Sargassum fluitans) was collected, mechanically pretreated, and digested in batch mode using ruminal fluid as inoculum. Two inoculum:substrate ratios (2:1 and 3:1, v/v) were operated for 7 days, and daily cumulative biogas production was recorded. The 3:1 ratio reached 10.6 mL of cumulative biogas, approximately twice the 5.0 mL obtained at 2:1, and its production curve did not plateau by day 7, suggesting ongoing activity. Elemental analysis of the sargassum showed a low C/N ratio (6.9:1) and high moisture (~95%), both of which constrain performance. Boyle’s model was used to estimate theoretical CH₄ and CO₂ yields and as expected, largely overpredicted the experimental volumes because it assumes ideal conversion. These results indicate that ruminal fluid enhances early-stage biogas formation but also highlight process limitations associated with biomass quality and short retention time. Future work should include extended digestion, co-digestion strategies to adjust the C/N ratio, and full monitoring of pH, soluble COD, VFAs, and volatile solids consumption.