首要城市（primate city）由地理学家 Mark Jefferson (1939)提出，是指全国最大城市的规模大幅超过其第二大城市 。这种城市往往有主导一个国家人口、政治、经济、文化的影响力。在发展中国家，这种都市较为常见，例如墨西哥首都墨西哥城、泰国首都曼谷、伊拉克首都巴格达、伊朗首都德黑兰、埃及首都开罗; 发达国家亦不乏此例，当中尤以欧洲最为普遍，包括英国首都伦敦、法国首都巴黎、希腊首都雅典、捷克首都布拉格、奥地利首都维也纳等。首位都市揭示出国家城市化水平差异大、城巿发展不平衡的现象 ，容易导致繁忙时间交通堵塞、各种污染、住屋不足等城市问题。

A primate city is the largest city in its country or region, disproportionately larger than any others in the urban hierarchy. A 'primate city distribution' is a rank-size distribution that has one very large city with many much smaller cities and towns, and no intermediate-sized urban centres: a King effect, visible as an outlier on an otherwise linear graph, when the rest of the data fit a power law or stretched exponential function. The 'law of the primate city' was first proposed by the geographer Mark Jefferson in 1939. He defines a primate city as being "at least twice as large as the next largest city and more than twice as significant." A primate city is number one in its country in most aspects, like politics, economy, media, culture and universities.