On-farm phosphorus (P) balances are of importance for identifying critical control points in P feeding and management. In this study, P mass balances were constructed on fourteen pasture-based dairy farms of Puerto Rico, using production and management data. The farms ranged in animal units (AU, 454.5 kg) density from 1.23 to 27.50, with a mean of 7.53 AU/ha. Dietary P concentrations ranged between 5.02 and 7.24 g P/kg dry matter intake (DMI), with a mean of 6.16 g P/kg. These dietary P concentrations averaged 87% higher than the National Research Council recommendation of 3.4 g P/kg DMI. High dietary P concentrations were not associated with higher milk yields (P > 0.05). Manure P concentration of 13.2 g/kg fecal dry matter was, unexpectedly, not related to dietary concentration or total P intake. Estimates of total annual P excreted per cow ranged from 23.9 to 36.9 with a mean of 30.3 kg. Phosphorus excretion levels were 55% higher than those recommended for maintenance and milk production. Soil test P (Olsen) levels showed that 87% of the paddocks receiving manure application exceed agronomic critical P levels of 35 mg/kg.The on-farm mass balance showed annual P surpluses that ranged from 15.0 to 472.9 with a mean of 156.2 kg/ha. Reducing the P concentration in the diet and in inorganic fertilizer applied to fields, and implementing best management practices, will have the greatest and most immediate impact on reducing the excess P present on dairy farms of Puerto Rico, and will contribute to the optimization of P use for eventual sustainable milk production and water quality maintenance.