PURPOSE: To use swept-source optical coherence tomography angiography (SS-OCTA) to investigate the alterations in retinal vascular density (VD) in patients presenting with congenital unilateral trochlear nerve palsy. METHODS: The medical records of patients diagnosed with congenital unilateral trochlear nerve palsy and those of a healthy control group were reviewed retrospectively. Comprehensive ocular examinations and SS-OCTA imaging were conducted. The study population was divided into three subgroups of eyes: paretic eyes, fellow eyes, and controls. All participants underwent comprehensive ophthalmic examinations, which included assessments of ocular motility, cycloplegic refraction, and axial length measurement. Various strabismus tests-the cover-uncover test, prism and alternate cover test, Krimsky test, and Parks-Bielschowsky test-were conducted to confirm the diagnosis and evaluate the extent of the condition. RESULTS: A total of 34 patients and 39 healthy controls were included. Sex and age distributions were similar between groups. Significant differences were observed in central VD of the superficial capillary plexus (SCP) and deep capillary plexus (DCP) between paretic eyes, fellow eyes, and control eyes. The fellow eye exhibited lower central VD compared to the paretic and control eyes. Inverse correlations between deviation angle and VD were noted in different SCP and DCP quadrants in paretic eyes. CONCLUSIONS: In our study cohort, there were significant changes in retinal VD in fellow eyes of patients with trochlear nerve palsy, which we speculate may be related to compensatory head position.