The increasing need to reduce agrochemicals has intensified the search for sustainable alternatives in crop production. Insect frass, a by-product of insect rearing, has recently emerged as a promising organic fertilizer. In the present study, the effects of Tenebrio molitor frass (TMF) on plant growth and productivity were evaluated in three vegetable crops, cucumber (cv. Aisopos), pepper (cv. Lamuyo), and lettuce (cv. Paris Island), under greenhouse conditions. Experimental plants were grown in pots under two substrate fertility levels (fertilized and non-fertilized peat, hereafter referred to as “rich” and “poor” soil) and received TMF at two rates (1% and 2% w/w), applied either once or twice. Plant height and weight, fruit number and weight, and total production per plant were recorded. TMF application, applied as a soil amendment, enhanced plant growth and yield of the treated plants compared to the control, although the magnitude and consistency of the response varied among crops, soil types, and measured parameters. A clear dose-dependent response was not observed, as the 2% rate did not consistently outperform the 1% rate. Likewise, splitting the same total amount of TMF into two applications did not significantly improve plant performance. The response to the TMF application varied among crops in terms of growth and yield parameters. Lettuce recorded the strongest response, while cucumber and pepper exhibited more moderate improvements. Notably, TMF significantly increased growth and productivity even at the lowest application rates under poor soil conditions. These findings demonstrate that TMF is a promising low-input organic fertilizer under the tested conditions and highlight the importance of optimizing application rate and strategy for sustainable vegetable production.