<p>Bio-stimulants play a pivotal role in the availability of nutrients and phytohormones and can substitute synthetic fertilizers in crop production. Current research aims to unfold the potential effect of seaweed extract (SWE) in improving onion bulb production. The various concentrations and application methods of SWE were evaluated to find the optimum concentration and suitable method of SWE application for maximum growth, yield, and quality of onion bulbs. Experiment was carried out in a Randomized Block Design, with three replications in four varying concentrations of SWE (C₁: 25%, C₂: 50%, C₃: 75%, and C₄: 100%) and two methods of application viz., M₁: root dipping of onion seedlings in SWE for 4&#xa0;h before transplanting, and M₂: the foliar application of SWE at 45, 60, and 75&#xa0;days after planting. Seaweed extract application improved leaf count, leaf length, highest leaf width, and plant height, with the lowest mortality. The phenological parameters, viz<i>.</i>, root length, root weight, neck diameter, equatorial diameter, polar diameter, and number of rings of bulbs, as well as yield parameters, viz., bulb fresh and dry weight, and bulb yield, were also substantially improved, with reduction in damaged bulbs and an enhancement in TSS, carotenoid, and chlorophyll content. Among various concentrations of SWE tested during the investigation, 50% SWE was potentially most effective. Foliar application of SWE was more effective than root dipping of onion seedlings. A potential productivity improvement was observed after foliar application of 50% SWE (1.60 times), followed by root dipping of 50% SWE (1.48 times).</p> Graphical abstract <p></p>