<p>This study was conducted to explore the effects of dietary protein restriction on growth performance, nutrient utilization, gut microbiota, and microbial metabolites in finishing pigs, as well as to elucidate potential sex-associated differences between gilts and barrows. A total of 36 gilts and 36 barrows at 110&#xa0;days of age were allocated to six groups in a 3 × 2 factorial arrangement consisting of three dietary protein levels and two sexes. Dietary crude protein levels were 17%, 15%, and 13% during phase I, and 15%, 13%, and 11% during phase II. The whole feeding trial lasted 51&#xa0;days. Overall, lowering dietary protein levels altered the digestibilities of crude protein, ether extract, calcium and phosphorus (<i>P</i> &lt; 0.01), and exerted sex-specific influences on serum total protein and nitric oxide concentrations (<i>P</i> &lt; 0.05). Among gut microbiota, five genera responded to protein restriction (<i>P</i> &lt; 0.05), eight showed sex differences (<i>P</i> &lt; 0.05), and six exhibited sex-specific responses to dietary protein levels (<i>P</i> &lt; 0.05). Dietary protein restriction reduced gut microbiota-derived ammonia nitrogen and six of eight biogenic amines (<i>P</i> &lt; 0.05); four of these metabolites differed between sexes (<i>P</i> &lt; 0.05), and four showed protein × sex interactions (<i>P</i> &lt; 0.05). The nitrogen-related metabolites were positively correlated with specific microbiota only in gilts (<i>P</i> &lt; 0.05). In conclusion, dietary protein restriction induced sex-specific alterations in gut microbiota and nitrogen-associated metabolites in finishing pigs, and such metabolic responses were exclusively linked to microbiota changes in gilts but not in barrows.</p>