This pilot study investigates the feasibility and assesses the impact of "Lactobacillus plantarum" PS128 probiotics on the neurological function in Rett syndrome. We conducted a randomized, double-blind, and placebo-controlled trial on Rett syndrome with MECP2 mutation aged between 1 and 50 years in Taiwan. In this pilot study, twice-daily "L. plantarum" PS128 or placebo was administered for 16 weeks. In addition to feasibility, we also assessed the changes utilizing the Mullen Scales of Early Learning. In total, 36 participants were finally randomized into "L. plantarum" PS128 (n = 18) or placebo (n = 18) groups. At the end of intervention, the retention rates were 100% for "L. plantarum" PS128 and 94.44% for placebo, with withdrawal rates of 5.56% for the placebo group. Both groups tolerated well, except for one "L. plantarum" PS128 participant who reported loose stool. The probiotic group showed a change of 2.19 ± 3.76, while the placebo group had -0.85 ± 5.09 (p = 0.051) in the total age-equivalent scores of Mullen Scales of Early Learning. There was a significant difference in the change of the total score on the Burke-Fahn-Marsden Movement Scale between probiotic group and placebo group (-12.19 ± 12.12 vs -4.59 ± 4.20, p = 0.020). In leg dystonia, the probiotic group exhibited a change of -4.11 ± 5.11 compared with -0.38 ± 1.50 in the placebo group (p = 0.008). Our findings affirm the feasibility of "L. plantarum" PS128 in Rett syndrome. Future clinical trials are mandatory to further explore its long-term impact on Rett syndrome.