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The study analyzed the characteristics, chemical composition, and in vitro gas production kinetics of <i>Porphyra umbilicalis</i> and <i>Saccharina latissima</i> silages. Each seaweed was ensiled in vacuum bags (three bags/silage) following a 2 × 3 factorial design, with two pre-treatments (unwilted or pre-wilted) and three silage types: unwashed seaweed ensiled without additive; seaweed washed and ensiled without additive; and seaweed washed and ensiled with 4 g of formic acid (FAC) per kg seaweed. Silages were kept for 3 months in darkness at 20 °C. Pre-wilting prevented (<i>p</i> < 0.001) effluent formation and reduced (<i>p</i> ≤ 0.038) the production of NH₃-N and volatile fatty acids for both seaweeds. Both pre-wilting and washing increased (<i>p</i> < 0.05) the ruminal degradability of <i>P. umbilicalis</i> silages but not of <i>S. latissima</i> silages. The pH of the FAC-treated silages was below 4.0, but ranged from 4.54 to 6.23 in non FAC-treated silages. DL-lactate concentrations were low (≤23.0 g/kg dry matter) and acetate was the predominant fermentation product, indicating a non-lactic fermentation. The estimated ruminal degradability of the <i>P. umbilicalis</i> and <i>S. latissima</i> silages was as average, 59.9 and 86.1% of that for high-quality rye-grass silages, respectively, indicating a medium-low nutritional value of these seaweed silages for ruminants.