In recent years, the poultry industry has actively sought more sustainable feed additives to address the various challenges of intensive production. To evaluate the oral effect with Lactobacillus reuteri CLP4 on performance, carcass traits, organ weights, cecal microbiology, tibial mineralization, blood indicators, and nitrogen and phosphorus excretion in broilers, 800 one-day-old unsexed Ross 308® chicks were randomly assigned to two treatments: a basal diet (T0; BD) without additives and BD+Lactobacillus reuteri CLP4 (T1) in the drinking water during the period from 0 to 21 days old. T1 decreased feed intake and feed conversion ratio without affecting body weight or viability of broilers. Likewise, it diminished abdominal fat and improved breast protein content. The probiotic also increased the relative weights of the thymus, spleen, liver, and pancreas, decreased the population of Salmonella spp. and cecal pH, and promoted the growth of lactic acid bacteria. Moreover, T1 improved serum immunoglobulin concentrations and decreased harmful serum lipids and nitrogen excretion, although without modifying the moisture, calcium, and phosphorus content of the tibia or phosphorus excretion. Oral supplementation with Lactobacillus reuteri CLP4 enhances productivity, breast protein, immune function, atherogenic index, cecal competitive exclusion, and reduces the environmental nitrogen load in broiler production.