Sponges are one of the most abundant benthic organisms in mesophotic coral ecosystems and support diverse microbial communities that play a role in key nitrogen processes. However, the specific contribution of individual sponges to reef nitrogen cycling remains poorly quantified. Here, we combined paired exhalant osculum and inhalant ambient water sampling with natural-abundance dual-isotope analyses of nitrate (δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻) and dissolved organic nitrogen (δ¹⁵N-DON) to characterize nitrogen cycling processes in eight mesophotic reef sponges of four genera sampled between 17 and 122 m depth in the Caribbean. A Bayesian isotope mass balance model was used to partition co-occurring nitrogen transformations: dissolved organic nitrogen production, ammonification, nitrification, and nitrate reduction from isotopic and concentration gradients between osculum and ambient water. Where model solutions were well-constrained, isotopic evidence points toward DON production and consumption, internal nitrification overprinting concurrent nitrate reduction, and concurrent NH₄⁺ enrichment in osculum water consistent with a predominance of dissimilatory nitrate reduction to ammonium (DNRA) over denitrification. Because DNRA conserves fixed nitrogen as NH₄⁺ rather than releasing it as N₂, these results suggest that sponges serve as a nitrogen retention mechanism in oligotrophic reef environments, which may influence reef-scale nitrogen budgets.