This paper describes an experimental and numerical investigation of a jet exhausting from a rectangular nozzle with an aspect ratio of 2 to examine jet noise reduction by fluid inserts. Fluid inserts constitute a noise reduction method that uses distributed air blowing into the nozzle, and that evolved from the hard-wall corrugation concept. This technique was investigated at the Pennsylvania State University and is well-established for round supersonic jets. However, its application to rectangular jets has not been studied sufficiently. Acoustic testing and numerical analyses were conducted to investigate the acoustic characteristics of a rectangular jet with fluid inserts. This paper discusses the difference between jets with hard-wall corrugations and those with fluid inserts, the effects of the injector arrangement, and the jet total temperature effect. The acoustic measurements reveal that the characteristics of acoustic benefits are different from those of the original hard-wall corrugations concept. It was shown that fluid inserts can provide approximately 3 dB acoustic benefits of the overall sound pressure level for a rectangular cold jet, and peak noise reduction and the broadband shock associated noise reduction were observed separately for the different injector pressure ratios. Moreover, it was found that the azimuthal directivity can be changed by the arrangement of the fluid inserts. The jet total temperature significantly influences the acoustic benefits. The numerical analyses, to investigate the difference in the flow field between the cold and hot jets, indicates that the interaction between a core jet and injector-induced flow strongly affects its acoustic characteristics. Because these characteristics are clearly different from the round jet results reported by previous studies, careful design is required for the application of fluid inserts to rectangular jets.