The Siamese Crocodile (Crocodylus siamensis) is critically endangered and, until a decade ago, few remaining wild populations were known to exist. Described here are the first in-depth surveys for C. siamensis in Laos with new field data on ecology and conservation. Small breeding populations of C. siamensis are confirmed to persist in Laos. During surveys between 2003 and 2008, C. siamensis was recorded in 13 sites of six river systems, where at least 36 individuals (1–11 per site) were documented. In all sites, crocodile densities and recruitment rates were extremely low. Eight nests were recorded—among the first wild nests of C. siamensis to be reported. Perennial, thickly vegetated floodplain lakes are critical dry-season refugia and breeding habitats for C. siamensis in Laos. Opportunistic collection of crocodiles by local communities was observed, and at all sites there is increasing degradation of floodplain lakes for agriculture or economic development. National crocodile records were compiled and indicate that, historically, C. siamensis was widespread in lowland riverine and palustrine habitats of Laos, with most records from Central and South Laos in the Mekong Plain. These records also suggest that a severe range decline has occurred over the past century, although most wetlands remain unsurveyed for crocodiles. Crocodylus siamensis is probably now extirpated from the Lao Mekong and many other wetlands. Remnant C. siamensis populations in Laos are of global importance. All documented breeding sites, and most confirmed national records, are in rural lands outside the national protected area system, and conservation efforts will require community-based approaches.