This study aimed to determine the protein and carbohydrate fractions as well as the in situ rumen degradability of Brachiaria decumbens silage (BDS) supplemented with soybean hulls. Five soybean hull inclusion levels were used: 0, 10, 20, 30, and 40% of the fresh matter of B. decumbens grass, distributed into a completely randomized design with five replications. The inclusion of soybean hulls caused a linear decrease (p < 0.001) in carbohydrate fractions A + B1 and a linear increase (p < 0.001) in carbohydrate fraction C. The percentage of non-protein nitrogen fraction increased linearly (p < 0.001), but the nitrogen fractions B1 + B2 and B3 presented a negative quadratic effect (p < 0.01) with soybean hull level and fraction C presented a linear decrease (p < 0.001). The dry matter (DM) degradability of soluble fraction (A) and the undigestible DM decreased linearly (p < 0.01) with the soybean hull level. The potentially degradable water-insoluble portion (DM fraction B) and degradability rate (c) of the DM fraction B increased linearly (p < 0.001) with soybean hull level. The crude protein (CP) fraction A presented a linear increase (p < 0.001) with soybean hull inclusion; however, soybean hull levels caused a linear decrease (p < 0.001) in the CP level of fraction B. The degradable insoluble fraction of NDF (D) of the silage increased linearly (p < 0.001) and the indigestible NDF fraction of the silage was linearly decreased with the soybean hull level (p < 0.001). The inclusion of intermediate levels (20–30%) of soybean hulls provided better protein and carbohydrate fractions and better quality of BDS.