There is ever-increasing interest in using natural fibers in polymer composite systems and textile industry. Prosopis juliflora fibers (PJFs) possess ideal characteristics that make them suitable for various applications. Alkali treatment of PJFs was primarily aimed to change their physico-chemical properties; 5% (w/v) NaOH concentration and 60 min of soaking time were found to be optimal. It is intriguing to note that optimally treated PJFs had higher cellulose (72.27 wt.%), lower hemicellulose (4.02 wt.%) and lignin (12.09 wt.%) contents, higher crystallinity index (73%), tensile strength, and thermal stability.