Food waste wastewater (FWW), defined in this study as the aqueous supernatant obtained after hydrothermal treatment and solid–liquid separation of food waste, is rich in soluble organic matter and nutrients and has considerable potential as a low-cost substrate for microbial cultivation. This study investigated the feasibility of using FWW to produce a liquid nitrogen-fixing bacterial (NFB) fertilizer. Five NFB strains were isolated from wheat rhizosphere soil, among which strain N4, identified as Arthrobacter pascens, exhibited the highest nitrogen-fixation capacity and was selected for fertilizer preparation. FWW produced at 120 °C provided the most suitable culture medium. Under the optimized conditions (100 rpm, 25 °C, pH 6, and 1% inoculum), the total bacterial count reached 1.38 × 1013 CFU mL-1. In a 35-day soybean seedling pot experiment, application of the FWW-derived NFB fertilizer increased the fresh weight of soybean plants by 73.6% compared with the CK treatment and was accompanied by increased soil available nitrogen and enzyme activities. The treatment was also associated with shifts in rhizosphere bacterial community composition.