With a growing global population and increasing needs for environmentally friendly food production, novel technologies and products are required to increase food production sustainably. In this regard, seaweed biostimulants are an innovative tool to improve crop growth and yield. However, seaweed biomass is typically obtained through wild harvest or collection of beach cast, with challenges around the consistency of quality and reliability of supply. Furthermore, common manufacturing methods can have issues with high chemical input, loss of potentially important compounds, and cost. Therefore, this thesis aims to develop a seaweed biostimulant utilising the cultivated seaweed Ulva stenophylloides and a novel low-input fermentation method. This research covers optimisation of fermentation parameters and detailed compositional characterisation of the resulting products (Chapter 2), and the quantification of the effect of selected ferments on growth and yield in plants in hydroponics systems (Chapter 3) and in potted soil (Chapter 4). The fermentation parameters: biomass loading, sucrose input, and incubation temperature were optimised. The initial sucrose input had the largest impact on fermentation progression and success, and was the main driver in resulting glucose content, total dissolved solids, pH, and pellicle yield. Excluding treatments with low biomass loadings, treatments with low and medium sucrose inputs reached completion and treatments with high sucrose inputs did not. As indicated by residual glucose, high biomass loadings provided valuable nutrients for microbial growth and high incubation temperatures were optimal for microbial growth. A range of nutrients were extracted from the seaweed biomass, but concentrations were insufficient as a sole nutrient source for plants based on chemical analyses. High biomass loadings were the main driver in the yield of protein and the sulfated polysaccharide ulvan, which both may modulate gene expression and induce metabolic changes in plants. High incubation temperature was the main driver for the yield of the auxin phenylacetic acid and indicates the production of other possible plant growth stimulating microbial biochemicals at high temperatures. Of the four ferments selected for further testing by ferment progression and chemical composition, ferment four (high biomass loading, mid-range sucrose loading, and high incubation temperature) was the best performing biostimulant tested on mung bean seedlings in hydroponics; at a 1 % (v/v) dose, plant fresh and dry weight were increased by 15 and 16 %, respectively, and the most root growth was generated. An auxin-like effect was not detected at any biostimulant dose. At a 1 % dose, an interactive effect with fertiliser was demonstrated in tomato seedling root number. A 2 % dose of biostimulant had adverse impacts on root growth over multiple root growth assays. When applied to potted tomato plants, inconclusive results were obtained in growth and yield due to sub-optimal application volumes and growing conditions that require further method development and retesting. Overall, this thesis advances knowledge in seaweed biostimulant manufacturing utilising U. stenophylloides with a fermentation production method. These results warrant further investigation into refinement of plant assays and growing conditions, the composition of the ferments through metabolomics and microbial identification, and the biostimulant effects on soil.