<p>Leguminous shrubs play a key role in forage-based tropical systems because of their potential for symbiotic biological nitrogen fixation (BNF). Although BNF in monocrop systems has been widely documented, quantitative field-based assessments under-mixed stands remain limited. This study evaluated BNF in <i>Leucaena leucocephala</i> (Lam.) de Wit grown in monocrop and intercropped with the non-leguminous shrubs <i>Guazuma ulmifolia</i> Lam. and <i>Moringa oleifera</i> Lam. A two-year field experiment was established using a randomized complete block design. Forage production, total nitrogen, and the proportion of nitrogen derived from the atmosphere (%Ndfa) were quantified across five harvests. BNF was estimated using the δ¹⁵N natural abundance method, with the non-leguminous shrubs serving as reference species. δ¹⁵N values in <i>Leucaena</i> were consistently lower than those of the non-leguminous reference species, indicating active BNF. Leaves and edible stems exhibited the most depleted δ¹⁵N signatures, with %Ndfa values ranging from 67.8 to 89.6% at the first harvest. Over the experimental period, total nitrogen fixed by <i>Leucaena</i> ranged from approximately 83 to 152&#xa0;kg&#xa0;N&#xa0;ha⁻¹, with greater system-level nitrogen inputs under monocropping conditions. Intercropping did not consistently increase the relative contribution of BNF, and reductions in total fixed N₂ were primarily attributable to lower <i>Leucaena</i> forage production, particularly when intercropped with <i>Moringa</i>. In contrast, the <i>Leucaena</i>–<i>Guazuma</i> association showed a more favorable forage production pattern than the <i>Leucaena</i>–<i>Moringa</i> association and was comparable to the Leucaena monocrop, with superior performance at some harvests. These findings highlight the capacity of <i>Leucaena</i> to sustain BNF under both monocrop and intercrop conditions and emphasize the importance of species selection and management objectives in the design of forage-based shrub systems.</p>