Black soldier fly (Hermetia illucens) larvae frass has gained increasing attention as a potential biofertiliser due to its nutrient content and role in organic waste recycling. This study evaluated the effects of black soldier fly larvae (BSFL) frass on the growth of Pak Choy (Brassica rapa L.) and selected soil chemical properties under rain-shelter conditions. Five fertiliser treatments were arranged in a completely randomised design with six replicates: NPK fertiliser (15:15:15), single frass application (BF1), double frass application (BF2), combined NPK with single frass application (NBF1), and combined NPK with double frass application (NBF2). Plant growth was assessed using shoot and root fresh and dry weights, while soil pH, total nitrogen, and available phosphorus were measured after a 30-day growth period. Data were analysed using analysis of variance followed by least significant difference tests. Fertiliser treatments significantly influenced plant biomass, soil pH, and available phosphorus, whereas soil total nitrogen did not differ significantly among treatments. The combined NPK and double frass treatment (NBF2) produced the highest shoot fresh weight, representing a 56.6% increase compared with NPK alone. Frass-only treatments resulted in biomass comparable to NPK fertiliser, indicating that BSFL frass was able to support Pak Choy growth under controlled conditions. Frass application also resulted in moderately higher soil pH, while available phosphorus declined in combined fertiliser treatments. Overall, BSFL frass shows potential as a supplementary organic fertiliser for short-cycle leafy vegetables; however, field-scale validation and longer-term nutrient dynamics require further investigation.