Following the colonial conquest, the French symbolically took over the Hà Nội Citadel, destroying what few buildings were left and replacing them with military barracks and depots. It has remained an army base ever since, though at the time of going to press some of its outer sections are being opened up to the public.

In order to make the colonists feel more at home and at the same time to reflect its vision of imperial grandeur, the French colonial government set about systematically rebuilding the major Vietnamese cities according to European specifications. Central to French urbanist plans was the segregation of these cities into quartiers, based primarily on the ethnicity of residents. In marked contrast to the relative squalor of their neighbours in the quartiers indigènes, the inhabitants of the European quarters of Hà Nội and Sài Gòn were given wide boulevards lined with spacious residential villas. Prior to 1920 many important public buildings were constructed in Parisian neo-classical style; the latter is best illustrated in Hà Nội by the architecture of St Joseph's Cathedral (1891), the Governor General's Palace (1906), the Palace and Office of the French Resident (1906), the High Court (1906), the Opera House (1911) and the Directorate des archives et bibliothèques (1917, now the National Library of Việt Nam); and in Sài Gòn by the Town Hall (1873, now the People's Committee Building), Notre Dame Cathedral (1880), the High Court (1885), the Vice-Governor's Palace (1890), the Central Post Office (1891) and the Opera House (1900).

By the 1920s a new hybrid architectural style known as 'Oriental' or 'Indochinese' had come into fashion. The leading exponent of this style was architect and city planner Ernest Hébrard, who in 1923 was appointed head of the Indochina Architecture and Town Planning Service (Service de l'architecture et de l'urbanisme de l'Indochine). Hébrard's idea was to incorporate into French architecture elements of indigenous design from the colonial territories of Việt Nam, Cambodia and Laos. Relatively few public buildings were constructed in this new style, but the Musée Louis Finot and École Française d'extrème orient (1925, now the Việt Nam History Museum), the University of Indochina (1926) and the French Financial Service Headquarters (1926, now the Ministry of Foreign Affairs) in Hà Nội stand out as excellent examples. In the south the Indochinese style was represented in Deleval's Musée Blanchard de la Brosse (1929, now the Việt Nam History Museum, Hồ Chí Minh City) and Dumail's Banque Nationale du Vietnam (1930). By the early 1940s the style had begun to find its way into residential architecture.

Art deco arrived during the late 1920s. In contrast to earlier colonial architectural styles, it was employed mainly in the construction of local government buildings, clinics and private houses and villas, as well as in exterior signage and interior furnishings. Hà Nội's imposing Banque de l'Indochine (1930), designed by Georges André Trouvé, is a notable exception. During the twilight of colonial rule the Indochina Architecture and Town Planning Service, now under Henri Cérutti-Maori, began constructing a number of buildings in the international modernist style, but this development was curtailed by the outbreak of the First Indochina War.

The spread of French architecture was by no means confined to Hà Nội and Sài Gòn. Impressive colonial-era structures may still be seen in many other centres of population, including Hải Phòng, Huế, Đà Nẵng and Nha Trang, whilst ruins of substantial French hill stations, razed to the ground during the conflicts of the 1950s, are evident at Sa Pa (Lai Châu Province), Tam Đảo (Vĩnh Phúc Province), Mẫu Sơn (Lạng Sơn Province) and Đà Lạt (Lâm Đồng Province).

Contact with the west brought few changes to Vietnamese Buddhist architecture, but oriental influence did find its way into French ecclesiastical architecture during this period, as indicated by the influence of Vietnamese traditional temple/pagoda design on the architecture of the remarkable Phát Diệm Cathedral (1875-1899) in Ninh Bình Province, 130 kilometres south of Hà Nội.

No survey of architectural developments of this period would be complete without reference to the Cao Đài Cathedral at Tây Ninh and its network of smaller Cao Đài temples which were constructed after 1926 in the wider region stretching from the Mekong Delta to Quảng Nam Province in central Việt Nam. The architecture of the cathedral itself represents an extraordinary yet harmonious fusion of eastern and western styles.