Seaweed extracts have been used as fertilizer for crops to enhance yield and quality of the crop products. Only very few studies have been carried out on the effects of seaweed extracts and beneficial soil microbes on growth of crop plants. Hence, the present study was conducted to investigate the synergistic effect of seaweed extract (SE) prepared from Sargassum wightii Greville, with and without the application of Rhizobium biofertilizer, on seed germination, seedling growth, biochemical constituents and yield of cowpea (Vigna unguiculata Walp Var. pusa 151 (family Fabaceae). Seaweed extract (1% concentration), with or without Rhizobium (biofertilizer) treatment, compared with the control, significantly (p≤ 0.05) enhanced vegetative growth (dry weight of shoot and root, number of lateral roots and total leaf area), biochemical parameters (total chlorophyll, carotenoids, proteins, lipids, total sugar and amino acids) and yield and yield components (pod number, length, weight, number of seeds per pod and 100 seeds weight). Seaweed extract application, along with Rhizobium biofertilizer, exhibited better results in terms of vegetative growth, biochemical and yield than the seaweed extract alone in enhancing the growth and yield of a cowpea.