Marine macroalgae are an increasingly attractive, locally renewable source of crop biostimulants, yet their agronomic value for rice grown under Bangladeshi conditions remains poorly quantified. We evaluated composite preparations of four seaweeds Caulerpa, Hypnea, Sargassum and Gracilaria on the growth and productivity of boro rice across two consecutive dry seasons (2022 and 2023) at Mymensingh. Seaweed was supplied either as a dried basal soil amendment (0–1.25 t ha-1) or as a foliar spray delivered at active tillering (0–1.25 L ha-1), in a randomized complete block design with three replications and 20 m² plots. In both seasons and under both delivery methods, seaweed improved grain yields relative to the untreated control. Basal application was most effective at 0.875–1.00 t ha-1, lifting grain yield to 8.8 t ha-1 in 2022 (Binadhan-24) and 5.73 t ha-1 in 2023 (BINA dhan25); the highest basal rate (1.25 t ha-1) depressed yield sharply, revealing a threshold beyond which the amendment became inhibitory. Foliar sprays raised grain yield by 16–19% over the control, with the optimum near 1.0–1.25 L ha-1. The response was driven mainly by more effective tillers, a greater number of filled grains and fewer sterile spikelets, whereas plant height and 1000-grain weight changed little a pattern indicating that seaweed enlarged the reproductive sink rather than individual grain size. Quadratic dose response models described a clear yield optimum, and grain yield was positively correlated with filled grains and effective tillers. Collectively, these results identify seaweed biostimulants as a low-cost, locally sourced input that can support sustainable intensification of boro rice.