The gut microbiota was reported to differ between children with autism spectrum disorder (ASD) and typically developing (TD) children, and dysbiosis of the gut microbiota in preterm infants is common. Here, we explored the characteristics of gut microbiota in children born preterm with ASD. We performed 16S rRNA gene sequencing using stool samples from ASD children born preterm and TD children born preterm. Alpha diversity was significantly greater in the ASD group. A comparison of beta diversity showed different clusters. Linear discriminant analysis effect size analysis revealed significantly more Firmicutes in the ASD group compared with the TD group. In conclusion, the gut microbiota in children born preterm differs between children with ASD and TD.