Background: Transradial artery (TRA) access for percutaneous coronary intervention (PCI) was associated with lower risks of major bleeding and vascular complications compared to transfemoral artery access. Use of large-bore (\ge7-Fr) guiding catheters through TRA approach increased the likelihood of radial artery occlusion (RAO). This study aimed to investigate whether use of the thin-walled 7-Fr Glidesheath Slender, allowing PCI with large-caliber guiding catheters, is superior to standard 7-Fr Cordis sheath with respect to periprocedural RAO within 24 hours after transradial coronary intervention (TRI) in complex lesions. Methods: A prospective randomized, controlled, single-blinded (patient-blinded) trial was conducted, randomizing 504 patients with TRI for complex lesions to either 7-Fr Glidesheath Slender or conventional 7-Fr Cordis sheath. The primary outcome was defined as the incidence of periprocedural RAO with Doppler ultrasound during the first 24 hours after TRI. Results: The incidence of early RAO was 10.3% for 7-Fr Glidesheath Slender and 13.5% for conventional 7-Fr sheath (p = 0.271). The procedural success rate for Glidesheath Slender was 92.9% and for Cordis sheath was 93.7% (p = 0.722). There was no signficiant difference between treatment arms in terms of local hematoma and radial spasm, whereas use of the Glidesheath Slender was associated with significantly less pain during the procedure (numeric rating scale [NRS], 2.27 \pm 0.75 vs. 2.45 \pm 0.95, p = 0.017). The assessment of radial artery in ultrasound parameters after complex TRI was improved with Glidesheath Slender. Conclusions: Among patients with complex coronary lesions undergoing TRI, 7-Fr Glidesheath Slender was not superior to conventional 7-Fr in the prevention of periprocedural RAO within 24 hours following complex PCI, without reducing RAO occurrence. Clinical Trial Registration: NCT04748068.