Trigonella foenum-graecum L. (fenugreek) belongs to family Fabaceae and has been used as medicinal herb since times immemorial. Fenugreek seed-alkaloid, known as trigonelline, controls diabetes through operation of the mechanism of insulin secretion, modulation of β cell regeneration and stimulation of activity of glucose metabolism related enzymes. Gamma-irradiated sodium alginate (ISA) elicits plant defence responses and biological activities in plants. A pot experiment was conducted to explore the effect of foliage-applied ISA (0, 40, 80 and 120 mg L−1) alone and in combination with soil-applied phosphorus (40 kg P ha−1) on growth, yield and quality attributes of fenugreek employing the soil deficient in phosphorus. Un-irradiated sodium alginate (UN 40 mg L−1) and de-ionized water were used as control. Of the treatments employed, 80 mg L−1 of ISA applied with 40 kg P ha−1 (P40) proved the best. It increased the chlorophyll content by 24.85 and 27.40%, carotenoids content by 15.00 and 23.52% at 60 and 90 days after sowing, respectively. Besides, it increased the seed yield by 131.0%, trigonelline content by 17.84%, trigonelline yield by 174.0%, seed alkaloid content by 32.98% and seed alkaloid yield by 208.64% over the control. Gel permeation chromatography of ISA revealed the formation of low molecular weight fractions which might be responsible for plant growth promotion in this study. Trigonelline content was determined by isocratic HPLC equipment (Model, LC-20AD).