Natural cellulosic fibers isolated from the plant sources have attracted growing attention in recent years, owing to their considerable potential to substitute synthetic fibers in fabrication of the plant fiber-reinforced polymer composites. However, it is extremely difficult to find the high-quality and sustainable raw materials, which requires a lot of searching and research. The Morus alba L. bark fiber (MABF) was isolated from the bark of the Morus alba L. plant, and subsequently treated with a 5 wt% alkali (NaOH) solution. To determine its potential as the alternative reinforcement for polymer composite applications, it is crucial to systematically investigate the chemical, morphological, wettability, thermal and tensile properties of the unmodified and alkali-modified MABFs. The experimental results demonstrate that the alkali modification resulted in a 21.31% increases in density, along with reductions of 15.63% in moisture absorption and 53.34% in water contact angles, respectively, compared with the unmodified fiber. Moreover, scanning electron microscope imaging reveals a rough surface morphology on the surface of the alkali-modified fiber. Fourier-transform infrared spectroscopy confirms the removal of the non-crystalline components. Alkali modification also improves the crystallinity indexes from 50.09% to 78.15%, the crystallite sizes from 1.23 nm to 3.02 nm, the thermal stability from 284.14 °C to 307.85 °C, the maximum thermal stability temperatures from 340.26 °C to 367.66 °C, and the kinetic activation energy ( E _a ) from 74.44 kJ mol ⁻¹ to 84.80 kJ mol ⁻¹ of the plant fiber, respectively. These findings indicate that the alkali modification effectively enhanced the comprehensive properties of the MABFs, establishing them as a promising and suitable candidate for the reinforcing lightweight polymer composites.