One meter and a half of floodplain sediment buries AD 14‐15th C brick temple ruins and cultural layers on the Laos floodplain of the Mekong, across from Chiang Saen Noi. Dark soil layers separating three flood‐sediment layers suggest the soils developed on sediment layers deposited by large floods. Little or no deposition occurs between such events. The upper flood layer was probably deposited by the September, 1966 flood that inundated Chiang Saen city to a depth of three meters, with maximum gage height reaching 13.82 m, and peak flow of 23,500 m3/s. In comparison, the depth of the August 2008 flood in the flood plain at the sample site was about 1.3‐m: maximum gage height was 10.57 m and peak flow ~15,000 m3/s. Although stage was at least 10 m for three days, the event did not deposit a recognizable sediment layer at the sampling site. In June 2012, during the annual low water period (stage = +2.45m at Chiang Saen), we sampled flood layers from the 11‐m high Laos riverbank for single‐grain quartz OSL dating. The top of the riverbank is equivalent to a river stage of 13.45 m. The floodplain, extending about 1 km to either side of the channel, presumably accumulates 0.35‐0.6m thick sediment layers during infrequent overbank floods that are 3‐4m deep across much of the floodplain. On the Thai side are the ruins of Chiang Saen Noi, founded in AD 1329 on similar banks about 10‐m above low water (described by Wood et al., 2008, Geomorphology, 101, 510‐523). Subsequently we obtained small‐aliquot quartz SAR OSL ages on floodplain silt 4‐7‐m deep of 4,750 ± 260, and 5,600 ± 310 years. Thus, this site began accumulating floodplain silt over bedload gravel of the former channel about 5,000 years ago. No clear flood layers occur in the surficial0.5‐m thick AD 14‐15th C cultural layer. The new Laos site is the best paleoflood record discovered to date for the upper Mekong River. Once dated, the stratigraphy will provide a better understanding of the frequency of large floods on the Mekong.