New proxy records from deep-sea sediment cores from the northwestern continental margin of Western Australian reveal a 5.3 million year (Ma) history of aridity and tropical monsoon activity in northwestern Australia. Following the warm and dry early Pliocene (~5.3 Ma), the northwestern Australian continent experienced a gradual increase in humidity peaking at about 3.8 Ma with higher than present-day rainfall. Between 3.8 and about 2.8 Ma, climate became progressively more arid with more rainfall variability. Coinciding with the onset of the Northern Hemisphere glaciations and the intensification of the Northern Hemisphere monsoon, aridity continued to increase overall from 2.8 Ma until today, with greater variance in precipitation and an increased frequency of large rainfall events. We associate the observed large-scale fluctuations in Australian aridity with variations in Indian Ocean sea surface temperatures, which largely control the monsoonal precipitation in northwestern Australia. Plain Language Summary Australia is the driest inhabited continent on the planet, with its moisture mostly sourced from the tropical monsoon in the north and the southern westerlies in the south. The continent has experienced large climate fluctuations in the geologic past, but long continuous records of paleoenvironmental changes are lacking, particularly prior to~0.55 Ma. Here, we address this paucity by presenting a continuous and fluctuating record of continental aridity and monsoonal activity in northwestern Australia since the Pliocene (5.3 Ma). These records are based on bulk-chemical X-Ray Fluorescence scans and particle-size distributions of the terrigenous fraction in two marine sediment cores from the NW Western Australian continental margin. A comparison with present-day sources of windblown and fluvial sediments taken near the NW Western Australian coast corroborates our interpretation of the terrigenous fraction in the marine sediment cores. We show how the northwestern part of the Australian continent has experienced large climate fluctuations since 5.3 Ma, expressed by large aridity contrasts and great changes in monsoonal precipitation that are driven by Indian Ocean sea-surface temperatures.