In comparison with the ever increasing literature on cross-border regions in the era of globalisation and regionalisation, less attention has been paid to paired border cities and their mutual relations. Moreover, existing studies on metropolises straddling international borders have tended to concentrate on North America and Western Europe, with little focus on cases in East Asia. Taking as a case study the first Special Administrative Region of China, Hong Kong, and its adjacent city Shenzhen, the first and most successful Special Economic Zone in China, this paper argues that the two border cities have integrated economically, socially and physically over the past two decades. In consequence, a cross-boundary metropolis is in the making. However, under the unique framework of 'One Country, Two Systems', this emerging cross-boundary metropolis has demonstrated some distinctive characteristics which have fundamental impacts on its development, governance and future prospects.