We aimed to compare the effectiveness of single- vs multiple-strain probiotics in a network meta-analysis of randomized trials.We searched MEDLINE, EMBASE, Science Citation Index Expanded, CINAHL, Scopus, Cochrane CENTRAL, BIOSIS Previews, and Google Scholar through January 1, 2019 for studies of single-strain and multi-strain probiotic formulations on outcomes of preterm, low-birthweight neonates. We used a frequentist approach for network meta-analysis and GRADE approach to assess certainty of evidence. Primary outcomes included all-cause mortality, severe necrotizing enterocolitis (NEC, Bell stage II or more), and culture-proven sepsis.We analyzed data from 63 trials involving 15,712 preterm infants. Compared with placebo, a combination of 1 or more Lactobacillus spp and 1 or more Bifidobacterium spp was the only intervention with moderate- or high-quality evidence of reduced all-cause mortality (odds ratio [OR], 0.56; 95% CI, 0.39-0.80). Among interventions with moderate- or high-quality evidence for efficacy compared with placebo, combinations of 1 or more Lactobacillus spp and 1 or more Bifidobacterium spp, Bifidobacterium animalis subsp, lactis, Lactobacillus reuteri, or Lactobacillus rhamnosus significantly reduced severe NEC (OR, 0.35; 95% CI, 0.20-0.59; OR, 0.31; 95% CI, 0.13-0.74; OR, 0.55; 95% CI, 0.34-0.91; and OR, 0.44; 95% CI, 0.21-0.90, respectively). There was moderate- or high-quality evidence that combinations of 1 or more Lactobacillus spp and 1 or more Bifidobacterium spp and Saccharomyces boulardii reduced the number of days to reach full feeding (mean reduction of 3.30 days; 95% CI, reduction of 5.91-0.69 days). There was moderate- or high-quality evidence that, compared with placebo, the single-species product B animalis subsp lactis or L reuteri significantly reduced duration of hospitalization (mean reduction of 13.00 days; 95% CI, reduction of 22.71-3.29 days and mean reduction of 7.89 days; 95% CI, reduction of 11.60-4.17 days, respectively).In a systematic review and network meta-analysis of studies to determine effects of single-strain and multi-strain probiotic formulations on outcomes of preterm, low-birthweight neonates, we found moderate to high evidence for the superiority of combinations of 1 or more Lactobacillus spp and 1 or more Bifidobacterium spp vs single- and other multiple-strain probiotic treatments. The combinations of Bacillus spp and Enterococcus spp, and 1 or more Bifidobacterium spp and S salivarius subsp thermophilus, might produce the largest reduction in NEC development. Further trials are needed.