Benzene, toluene, ethylbenzene, and xylene (BTEX) are of great environmental concern because of their widespread occurrence in groundwater and soil, posing an increasing threat to human health. The aerobic denitrifying BTEX-degrading bacterium Pseudomonas thivervalensis MAH1 was isolated from BTEX-contaminated sediment under nitrate-reducing conditions. The degradation rates of benzene, toluene, ethylbenzene, and xylene by strain MAH1 were 4.71, 6.59, 5.64, and 2.59 mg·L−1·day−1, respectively. The effects of sodium citrate, nitrate, and NaH2PO4 on improving BTEX biodegradation were investigated, and their optimum concentrations were 0.5 g·L−1, 100 mg·L−1, and 0.8 mmol·L−1, respectively. Moreover, MAH1, which has nirS and nosZ genes, removed ammonium, nitrate, and nitrite at 2.49 mg NH4+-N·L−1·h−1, 1.50 mg NO3−-N·L−1·h−1, and 0.83 mg NO2−-N·L−1·h−1, respectively. MAH1 could help in mitigating the pollution caused by nitrogen amendments for biostimulation. This study highlighted the feasibility of using MAH1 for the bioremediation of BTEX-contaminated sites.