Objectives A gradually increasing number of studies have reported that dysregulation in the immune system may be a contributing factor in the etiopathogenesis of autism spectrum disorder (ASD). However, only limited data has existed about the role of tumor necrosis factor (TNF) superfamily members in ASD. This study aimed to investigate whether serum levels of BAFF, APRIL, TWEAK, TNFSF18 TNFR2, and TNFRS12A are associated with ASD in preschool children. Methods A total of 65 (52 boys/13 girls) preschool children with ASD and 40 (28 boys/12 girls) healthy controls were included in the study. The severity of the ASD symptoms was assessed by the Autism Behavior Checklist and the Childhood Autism Rating Scale. Serum levels of immunologic parameters were measured by enzyme-linked immunosorbent assay. Results Analysis indicated that serum BAFF, TNFSF18, TNFR2, and TNFRS12A levels were significantly lower and serum APRIL and TWEAK levels were significantly higher in the ASD group than in the control group. However, with logistic regression only low BAFF levels and high APRIL levels had relationships with the presence of ASD. Conclusions These findings suggest that TNF superfamily members, especially the BAFF/APRIL system may play a role in the pathophysiology of ASD.