This study aimed to estimate the association of P2O5 doses with and without rhizobia inoculation to maximize the agronomic performance and yield of cowpea on an Oxisol in the Cerrado region of Tocantins. The experiments were carried out in two periods during the 2014/2015 growing season at the Experimental Farm of Gurupi belonging to the Federal University of Tocantins, using the variety BRS Novaera. Inoculation was carried out with a standard strain recommended for cowpea. The experimental designs were randomized in blocks in a 2×6 factorial arrangement, with the first factor consisting of the inoculation (presence and absence) and the second factor consisting of six phosphorus doses (0, 30, 60, 90, 120, and 150 kg ha−1 of P2O5), totaling 12 treatments with four replications. Total chlorophyll, leaf P content, flowering, hundred-grain weight, and yield were evaluated. Phosphorus fertilization had a positive influence on the total chlorophyll, flowering, and leaf P content of cowpea on the Oxisol in the Cerrado region of Tocantins. The application of 113.42 kg ha−1 of P2O5 provided a yield of 1,124 kg ha−1 in the first planting period, corresponding to an 83% increase. However, phosphorus fertilization and inoculation increased grain yield in the second period, with the maximum yield reaching 145.94 kg ha−1, obtained with the maximum effect dose of 123.04 kg ha−1 of P2O5, with a yield gain of 257.3%.