This study investigated the synergetic relationship between hydrophobicity and bulk density of torrefied mango seed kernel (MSK) as a low-cost biosorbent for water and wastewater treatment. MSK was torrefied at 200 to 300°C and characterized for mass loss, bulk density, and hydrophobicity using standard methods. Hydrophobicity was determined using the water drop penetration time (WDPT) test. Also surface functional groups of untorrefied and torrefied MSK were characterized using Fourier Transform Infrared Spectroscopy (FTIR). Results showed that mass loss increased linearly with torrefaction temperature, from minimal losses at 200 to 225°C (3.33-4.67%) to substantial losses at 250 to 300°C (48.67 to 62.33%), indicating progressive devolatilization and carbonization. Bulk density exhibited a segmented response, increasing at low torrefaction temperatures (≤ 225 °C) due to limited degradation, but decreasing markedly at higher temperatures (250 to 300°C) with a strong inverse correlation to temperature (r = - 0.91) , attributed to pore development and structural breakdown. All torrefied samples were classified as extremely hydrophobic, with WDPT values ranging from 25,880 to 34,105 seconds. Hydrophobicity showed stronger sensitivity to torrefaction temperature than bulk density. FTIR spectra confirmed the progressive loss of oxygen-containing functional groups (O-H, C = 0 , and C-O) with increasing temperature, explaining the enhanced hydrophobicity. Overall, torrefaction produced a stable, highly hydrophobic MSK biosorbent with reduced bulk density at higher temperatures, highlighting hydrophobicity as a more critical parameter than bulk density in optimizing torrefaction conditions for sustainable biosorbent activation.