The objective of this study was to evaluate the effect of substituting urea and rice bran with mulberry or a mixture of mulberry and Leucaena in the diet on growth rate, feed intake, nutrient digestibility, nitrogen balance, rumen fermentation, estimated methane production, and rumen microbial protein synthesis in lambs fed a basal diet of urea-treated rice straw. The experiment consisting of three supplement treatments with six replications was arranged in a randomized block design. Eighteen lambs with an initial body weight of 14.4 ± 3.35 kg were used. The three supplement treatments were (i) urea–rice bran (UR) comprising 38.5% of the diet, (ii) mulberry foliage comprising 30% of the diet as UR substitution (Mb), and (iii) a mixture of mulberry–Leucaena foliage in a 1:1 ratio comprising 30% of the diet as UR substitution (MbL). All lambs were fed a basal diet of urea-treated rice straw, and the diets were formulated with iso-energy and iso-protein content. Substituting urea rice bran with either mulberry foliage or a mixture of mulberry–Leucaena foliage yielded similar effects on urinary and faecal nitrogen excretions, nitrogen retention, rumen fermentation, microbial nitrogen yield, and the average daily gain of lambs (71.4 g/day). Both the mulberry and the mulberry–Leucaena mixture supplements exhibited higher dry matter intake by 15.4% and 9.9% and neutral detergent fibre digestibility by 17.5% and 14.5%, respectively, compared to urea–rice bran supplementation. These findings indicate that mulberry or a mixture of mulberry–Leucaena foliage is a promising alternative to replace urea–rice bran in improving the nutritional values of the rice straw basal diet for sheep.