Preterm birth (<37 weeks’ gestation) has been associated with problems in social functioning. Whether social inhibition is specifically related to preterm birth and whether early parenting may protect against social inhibition difficulties is unknown. To explore effects of gestational age and early parent–infant relationships on social inhibition, 1314 children born at 26–41 weeks gestational age were studied as part of the prospective Bavarian Longitudinal Study. Early parent–infant relationship quality was assessed postnatally with the parent–infant relationship index. Social inhibition was assessed at age 6 years using an experimental procedure, in which nonverbal and verbal responses were coded into social inhibition categories (disinhibited, normally responsive, inhibited). Multinomial logistic regressions indicated that children with lower gestational age showed more socially disinhibited (nonverbal: OR = 1.27 [95% CI = 1.17–1.40], verbal: OR = 1.23 [95% CI 1.13–1.35]) and inhibited (nonverbal: OR = 1.21 [95% CI = 1.11–1.32], verbal: OR = 1.11 [95% CI = 1.01–1.21]) responses. Good early parent–infant relationships were associated with less verbal disinhibition (OR = 0.70 [95% CI = 0.52–0.93]). Findings suggest that children with lower gestational age are at greater risk to be both socially inhibited and disinhibited. Early parenting affected risk of abnormal social responses. Supporting early parent–infant relationships may reduce preterm children’s risk for social difficulties.