The study aimed to evaluate the nutritional value of watermelon plant silage (WPS) and to assess the effect of replacing alfalfa hay (AH) with WPS in a diet formulated for dairy goats on the basis of two in vitro trials. The study used four goats as donors of rumen microbiota to analyze in vitro fermentation parameters. The chemical composition analysis revealed that WPS and AH had similar protein content (21.1 vs. 18.9 g CP/100 g DM), but WPS had lower contents of organic matter (77.4 vs. 89.6 g/100 g DM), neutral detergent fiber (36.3 vs. 49.4 g/100 g DM), acid detergent fiber (26.4 vs. 34.6 g/100 g DM), acid detergent lignin (6.40 vs. 8.01 g/100 g DM), and amino acids (716 vs. 874 g AA/kg N) compared to AH. Conversely, WPS had higher crude fat content (3.09 vs. 1.29 g/100 g DM) and different macro- and micromineral content values than AH. In a first in vitro trial, WPS and AH were incubated independently to compare their fermentation behaviour, indicating that the metabolizable energy tended to be higher for WPS compared to the AH diet (6.67 vs. 5.72 MJ/kg DM; P = 0.058). WPS fermentation produced higher concentrations of total volatile fatty acids than AH (66.6 vs. 53.3 mM; P = 0.037), lower proportions of propionate and valerate (P ≤ 0.022), higher proportions of isobutyrate (P = 0.001), and showed similar gas production (GP) kinetics, as degradation rate and its potential release, and estimated organic matter digestibility. In a second in vitro trial, a formulated goat diet (consisting of commercial concentrate and AH in a 1:1 ratio) was used as a control to assay the impact of replacing 25 and 50 % of AH by WPS. Trends were observed in several parameters, such as a linear increase in GP rate (P = 0.088), organic matter digestibility (P = 0.067), and CH4 concentration in the gas produced (P = 0.094) as AH was replaced by WPS. The study concluded that WPS could serve as a viable fodder to replace AH in conventional dairy goat diets while simultaneously reducing agricultural waste and serving as a regenerative model for implementing the circular economy strategy in the affected agronomic sectors.