In this study, a combination of phosphorus (PP) oxoanions in a submerged plasma irradiation (SPI) system was used to enhance the removal efficiency of dyes from wastewater. The SPI system showed synergistic methylene blue removal efficiency, due to the plasma irradiation and Fenton-like oxidation. The ferrous ions released from the iron electrode in the SPI system under plasmonic conditions form complexes with the PP anions, which can then react with dissolved oxygen (O2) or hydrogen peroxide (H2O2) via Fenton-like reactions. The experimental results revealed that a sodium triphosphate (TPP) combined SPI system has a higher dye removal efficiency than a tetrasodium pyrophosphate (DP) or a sodium hexametaphosphate (HMP) combined SPI system under similar dissolved iron ion concentrations. To confirm the accuracy of the proposed removal mechanism via Fenton-like oxidation, it was compared to SPI systems under an oxygen environment (TPP/SPI/O2 (k = 0.0182 s−1)) and a nitrogen environment (TPP/SPI/N2 (k = 0.0062 s−1)). The results indicate that the hydroxyl radical (radical dotOH) in the TPP/SPI/O2 system is the major oxidant in methylene blue removal, because the dye degradation rates dramatically decreased with the addition of radical scavengers such as tert-butanol (k = 0.0023 s−1) and methanol (k = 0.0021 s−1). On the other hand, no change was observed in the methylene blue removal efficiency of the TPP/SPI/O2 system when it was subjected to a wide range of pHs (3–9). In addition, it was proved that this system could be used to eliminate six different commercial dyes. The results of this study indicated that the TPP/SPI/O2 system is a promising advanced oxidation approach for dye wastewater treatment.