Children born extremely preterm, especially those with intraventricular haemorrhage (IVH), are at increased risk of adverse cognitive outcomes during childhood. The present study aimed to explore the effects of IVH (grades I–II) on hippocampal volumes, and their correlates with cognitive performance. The sample consisted of 94 participants, including 54 children born extremely preterm (19 with IVH, grades I–II), and 40 children born at term. All participants underwent a magnetic resonance imaging study at the age of 10 (Mage= 10.20 years; SDage= 0.78), and 74 of them (45 extremely preterm and 29 full-term) carried out a cognitive assessment at 12 years old. Children born extremely preterm had lower scores in cognitive performance compared to their full-term peers. Significant positive partial correlations were observed between global bilateral hippocampus, left CA-field, and left subiculum volumes with processing speed in the full-term group, while no significant correlations were found in the extremely preterm group. Moderation analyses in the extremely preterm sample revealed that low-grade IVH moderated the relationship between right hippocampal volume and full-IQ (F(4,40) =5.42,p= 0.001,R2= 0.35). Having greater right hippocampal volume had a protective effect on full-IQ in those children born extremely preterm with low-grade IVH.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00429-023-02643-w.