Background <p>Breast milk is the optimal infant nutrition, and its composition is influenced by maternal factors including supplement intake. This study aimed to investigate the effects of postpartum probiotic supplementation on breast milk composition and infant health outcomes.</p> Methods <p>This double-blind, randomized, placebo-controlled trial enrolled 56 mother‑infant pairs. Using SPSS software-generated sequences, 56 pairs of healthy mothers and infants were randomly assigned in a 1:1 ratio to receive either a 30‑day probiotic supplement (<i>Bifidobacterium</i>, <i>Lactobacillus bulgaricus</i>, and <i>Streptococcus thermophilus</i>) or an identical-appearing placebo. Neither the investigators, nursing staff, nor outcome assessors were aware of the group assignments. The primary outcome measures were changes in breast milk nutritional components (energy, protein, lipids) and antioxidant status (total antioxidant capacity-TAC, malondialdehyde-MDA). Infant growth parameters (length, weight, head circumference) served as key secondary outcome measures.</p> Results <p>All 56 participants (28 per group) were included in the analysis. Compared to the placebo group, the probiotic group demonstrated significant increases in breast milk energy (<i>P</i> &lt; 0.05), protein (<i>P</i> = 0.04), and lipid content (<i>P</i> &lt; 0.05). Compared to the placebo group, the probiotic group had significantly higher TAC levels, indicating enhanced antioxidant capacity, and lower MDA levels, indicating reduced oxidative stress. Infants in the probiotic group also had significantly greater weekly gains in length (<i>P</i> &lt; 0.001), weight (<i>P</i> &lt; 0.001), and head circumference (<i>P</i> &lt; 0.05).</p> Conclusion <p>This pilot study provides preliminary evidence that postpartum probiotic supplementation may be associated with beneficial changes in breast milk composition and antioxidant status.</p>