Invasion of native communities by alien species is one of the main threats to biodiversity. This threat is particularly high on isolated tropical islands, with the Hawaiian Islands being one of the foremost examples of this phenomenon. The Kīpahulu Biological Reserve on the island of Maui provides a unique opportunity to study how alien plant species establish and spread into native communities. The reserve covers a wide altitudinal range and has been surveyed multiple times. We analyzed the spatiotemporal dynamics of alien plant species invasion in the forest of the valley between 1945 and 2018. At the time the reserve was created, most of the valley was relatively free of alien plant species. Extensive management, including fencing to exclude ungulates, was conducted during the 1980–1990s to stop the invasion by alien plant species occurring since the 1970s. Today, only the forest located in the upper part of the valley (>1400 m) is nearly free of alien plant species. Despite management efforts, the middle portion of the valley (between 960 and 1400 m) is in the process of being invaded by Clidemia hirta (Melastomaceae), Hedychium gardnerianum (Zingiberaceae), and other invasive plants. Although C. hirta and H. gardnerianum were detected at an incipient stage in the 1980s, those species invaded a large part of the Kīpahulu valley. This highlights the importance of control of alien plants following early detection, especially in areas where initial numbers and densities are low. Additional management efforts are urgently needed to limit alien species invasions into one of the last relatively pristine wet forests of Hawai‘i.