Purpose Today, coronavirus disease-19 (COVID-19) treatment is an evolving process, and synbiotic administration has been suggested as a new therapeutic strategy. This study aims to investigate the effect of synbiotic supplementation in COVID-19 patients. Design/methodology/approach In this placebo-controlled trial, 80 patients were randomized to receive oral synbiotic capsule (containing fructooligosaccharide and seven bacterial strains; Lactobacillus (L) casei, L. rhamnosus, Streptococcus thermophilus, Bifidobacterium breve, L. acidophilus, Bifidobacterium longum, L. bulgaricus, each one 109 colony-forming units) or placebo for two months. Inflammatory markers (Interleukin-6 [IL-6], C-reactive protein [CRP], erythrocyte sedimentation rate [ESR]) and white blood cell (WBC) count were evaluated at two timepoints (baseline, two months later). The measured variables were adjusted for confounders and analyzed by SPSS v21.0. Findings All 80 enrolled patients completed the study. The study adherence was good (approximately 70%). The mean changes for IL-6 were not significant (Δ = −0.6 ± 10.4 pg/mL vs Δ = +11.2 ± 50.3 pg/mL, p > 0.05). There were no significant improvements for CRP, ESR and WBC. Originality/value Administration of synbiotics for two months did not improve inflammatory markers in COVID-19 patients.