Herein, a denitrifying bacterium that produced greenish fluorescent pigment under aerobic conditions was accidentally isolated from municipal sewage sludge. Using 16S-rDNA sequence analysis, we identified the isolate as Pseudomonas aeruginosa R12, with 100% similarity. We achieved the highest pigment production rate (1.36 mg/L/h) in a 1-L bioreactor under aerobic conditions, using the optimal culture parameters determined in this study: $37^{circ}C$, pH 8.0, 200 rpm, 5 vvm aeration, and medium containing succinate and $(NH_4)_2SO_4$. The pigment was not a secondary metabolite and had no antibacterial activity on its co-isolates. Under anaerobic conditions, the isolate produced mainly $N_2$ and behaved as a strong denitrifier, displaying synergistic denitrification with co-isolated denitrifiers. To our knowledge, herein we have described the first instance in which P. aeruginosa R12 produces a fluorescent pigment under aerobic conditions. This newly-isolated strain therefore shows potential as a commercial resource for natural pigment.