BACKGROUND: Recent studies have identified diagonal conduction block lines at the electrophysiologically defined superior vena cava (SVC)-right atrium (RA) junction using three-dimensional mapping systems and reported the utility of SVCI using these lines. However, these block lines are not always observed, requiring the use of a conventional horizontal isolation approach. OBJECTIVE: This study aimed to visualize the electrophysiologically defined SVC-RA junction using high-resolution mapping with atrial extrastimulus pacing. METHODS: Twenty-five patients underwent high-resolution mapping during sinus rhythm (Sinus map) and extrastimulus pacing (S2 map) using a three-dimensional mapping system (CARTO 3, version 8). Slow conduction or conduction block along the SVC-RA junction was assessed by adjusting the lower threshold cut-off value in the "early meets late" analysis. The length of slow or conduction block lines was compared between the two maps. RESULTS: The S2 map revealed significantly longer slow or conduction block lines compared to the Sinus map (52.5 ± 18.0 mm vs. 35.6 ± 16.1 mm, p = 0.0013). The adjusted lower threshold values were comparable between the two maps: 26.4 ± 3.7% for the S2 map and 25.2 ± 3.8% for the Sinus map (p = 0.2819). In the S2 map, a slow or block line covering more than half of the junction was observed in all patients. Furthermore, the sinus node was consistently located inferior to this line. CONCLUSION: High-resolution mapping with atrial extrastimulus pacing more clearly visualizes the SVC-RA junction, potentially enabling more effective SVCI and estimation of the sinus node location.