Background: The thoracic retrolaminar block (TRLB) is a relatively new regional analgesia technique that can be used as an alternative to the thoracic paravertebral block. This study aimed to evaluate the postoperative analgesia effects of ultrasound-guided TRLB in children undergoing open cardiac surgery via median sternotomy incision. Methods: Sixty-six patients aged 2–8 years were recruited. In the TRLB group, 0.25% bupivacaine 0.4 ml/kg was injected into the retrolaminar space on both sides at the level of the T4 lamina. Patients in the control group were injected with 0.9% saline. The primary outcome measure was fentanyl consumption in the first 24 h post-extubation. The secondary outcome measures were the total intraoperative fentanyl consumption, postoperative modified objective pain score (MOPS), and time to extubation. Results: The total intraoperative fentanyl requirements and fentanyl consumption in the first 24 h post-extubation were significantly lower (P < 0.001) in the TRLB group (9.3 ± 1.2; 6.9 ± 2.1 μg/kg, respectively) than in the control group (12.5 ± 1.4; 16.6 ± 2.8, respectively). The median (Q1, Q3) time to extubation was significantly shorter (P < 0.001) in the TRLB group (2 [1, 3] h) than in the control group (6 [4.5, 6] h). The MOPS was significantly lower (P < 0.05) in the TRLB group than in the control group at 0, 2, 4, 8, 12 and 16 h post-extubation. Conclusions Bilateral ultrasound-guided TRLB is effective in providing postoperative analgesia in children undergoing open cardiac surgery via median sternotomy incision.