The influence of alkali treatment and fiber percentage on the physical, mechanical and thermal properties of hemp fiber reinforced gypsum composites are examined. Short hemp fibers (15 mm in length) were subjected to alkali treatment using 5, 10, and 15 wt. % NaOH concentrations for 5 h. Both untreated and NaOH treated hemp fiber reinforced gypsum composites were fabricated with fiber percentages of 3, 6, 9, 12, and 15 vol. % through the hand layup process. The fabricated composite samples were evaluated for density, water absorption, compressive strength, flexural strength, and thermal conductivity. The composite with 10 wt. % NaOH treated fibers, and 12 vol. % fiber content showed significant improvements, with a 337 % increase in flexural strength, a 136 % increase in flexural modulus, and a 109.8 % increase in compressive strength compared to pure gypsum. The results for thermal conductivity indicate that NaOH concentration has a minimal effect on enhancing thermal resistance compared to the influence of fiber percentage. SEM analysis of untreated and treated fibers revealed surface modifications and changes in fiber characteristics due to NaOH treatment. The results showed that stronger NaOH treatments and higher fiber content negatively impacted fiber suitability, stiffness, and overall composite properties.