In this work, the viability of the probiotic bacterium Lactobacillus fermentum HA6 isolated from naturally fermented vegetables in Vietnam was improved by growing the bacterium into a mild acid condition (pH 4.0). Viability and probiotic functionality [X-prolyl dipeptidyl aminopeptidase (PepX) activity] of the acid-adapted bacterium exposed to simulated gastrointestinal conditions were investigated. After 180 min in the simulated gastric juice (0.3 g/L pepsin, pH 2.0), the viability of acid-adapted L. fermentum HA6 (11.5%) was higher than that of control L. fermentum HA6 (2.2%). Specific PepX activity of acid-adapted cells (24.5 U/mg) was higher than that of control cells (17.8 U/mg). After 180-min exposure to the simulated small intestinal medium (0.3 g/L bile salts, 0.1 g/L pancreatin, pH 8.0), the viability of acid-adapted L. fermentum HA6 (13.5%) was twofold as high as that of control L fermentum HA6 (8.0%). Our results suggested that acid adaptation has a key role in acquiring cross-protection mechanism, which in this study resulted in higher survival of L. fermentum HA6 after simulated gastrointestinal stresses. The strategy of acid adaptation could be valuable for the production of robust probiotics.