Abstract: Promoting balanced regional development between northern and southern China is essential for achieving coordinated national growth and a key component of advancing Chinese-style modernization. This paper explores the widening North-South gap in high-quality urban economic development in China, particularly in recent years. Using panel data from 280 Chinese cities between 2006 and 2021, the study examines the spatiotemporal evolution and underlying mechanisms of this gap from the perspective of high-quality urban economic development. The findings reveal that over the study period, the level of high-quality urban economic development in China has steadily improved, with a distinct spatial pattern of “higher in the south, lower in the north; faster in the south, slower in the north.” Notably, after 2013, the North-South disparity has become more pronounced. Decomposition analysis of regional differences indicates that regional variations are the primary source of the North-South gap in high-quality urban economic development, and the contribution of these regional differences has been growing. The QAP (Quadratic Assignment Procedure) analysis highlights that differences in economic development fundamentals, economic structure, and development outcomes are the key drivers of this gap, while disparities in economic development security have a relatively minor impact. Furthermore, the driving factors in the North and South exhibit divergent trends. Based on these findings, the paper offers policy recommendations aimed at narrowing the gap in high-quality economic development between northern and southern cities. These suggestions aim to build a more balanced and sustainable pattern of high-quality economic development across China’s urban regions.