The experiment was conducted to evaluate the effects of dietary supplementations with S. platensis (purity ≥ 99%) and probiotics (1.0 × 109 CFU/g B. subtilis and 1.0 × 108 CFU/g L. plantarum) on pigs’ growth performance, immune responses and fecal Lactobacillus spp. and E. coli contents. Eighty weaned piglets (8.85 ± 0.20 kg) with negative maternal food-and-mouth disease (FMD) antibodies were blocked by initial weight, sex, and litter of origin and subsequently randomly allotted to 1 of 4 dietary treatments, consisting of 5 replicates of 4 pigs each. In this experiment, a control diet (CON), diet containing 0.5% S. platensis (SP) or 0.05% probiotics (PRO) or diet combined with 0.5% S. platensis and 0.05% probiotics (SP + PRO) was fed to weaned piglets for 28 days. On d 8, pigs were immunized with type O and A FMD vaccine. Added S. platensis, probiotics or both of them increased (p < 0.001) the average daily gain and average daily intake, decreased (p ≤ 0.015) the feed conversion ratio of weaned piglets, but had no effect on the antibody titer of type O and A FMD (p ≥ 0.22). Supplemented S. platensis, probiotics or both of them increased the contents of IgG on d 28 (p < 0.001), IgM on both d 7 and 28 (p ≤ 0.041), IgA and complement 3 on d 28 (p ≤ 0.039), but showed no impact (p ≥ 0.054) on complement 4, toll-like receptor 3 (TLR3) or TLR4 on both d 7 and 28. Unlike probiotics, S. platensis-supplement showed no influence (p ≥ 0.34) on the contents of Lactobacillus spp. and E. coli of pig feces at 7 and 28 days of the trail. The synergistic effect between S. platensis and probiotics only be detected (p ≤ 0.024) on Ig M on d 7 and 28, IgG and complement 3 on d 28. These results indicated that adding 0.5% S. platensis, 0.05% probiotics or both of them when immunized with FMD vaccine on weaned piglets can promote their growth performance and enhance their serum IgG, IgM, IgA, and C3. However, S. platensis was weaker than probiotics in improving the fecal flora of weaned piglets.