The little explored Cambodian and Vietnamese Phuquoc–Kampot Som Basin is a Late Jurassic to Early Cretaceous foreland basin developed in response to the build-up of a palaeo-Pacific magmatic arc. A combination of seismic data, well data and outcrop geology complemented by fission track and U/Pb analysis is used to unravel the basin history. This reveals a hitherto unknown earliest Palaeogene basin inversion associated with the Luconian suturing to SE Asia and the shutdown of palaeo-Pacific subduction underneath SE Asia. The Phuquoc–Kampot Som Basin and the Khorat Basin in Thailand constitute the erosional remnants of a larger basin that covered large parts of SE Asia in Late Mesozoic time, and subsequently became segregated during earliest Palaeogene inversion and erosion. Inversion was focused along the several hundred kilometres long Kampot and Khmer–Chanthaburi fold belts that confine the Phuquoc–Kampot Som Basin and merge with the Mae Ping and the Three Pagodas fault zones. These connections, together with local NW–SE-trending sinistral transpressional faults offshore, indicate a link between initial SE Asian left-lateral strike-slip faulting and the Luconian suturing. The separation between the once unbroken Khmer–Chanthaburi Fold Belt and the Phetchabun Fold Belt in Thailand suggests a 50–100 km Cenozoic left-lateral offset across the Mae Ping Fault Zone.