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The spatial extent, duration, and depth of inundation are key factors in the functioning of floodplain ecosystems. Satellite-based methods have been developed for mapping flooding extent and water surface height across large floodplain reaches, but on the Amazon floodplain, ground measurements for rigorous validation of these products currently exist for only a few instrumented sites. Here we report results of a field test of networks of Thermochron iButtons® low-cost temperature logging devices for documenting dates of the onset and recession of floodwaters during the 2008-2009 flood cycle at two sites on the Amazon floodplain. Of 237 iButtons deployed on tree trunk gauges at the Mamirauá and Piagaçu Sustainable Development Reserves on the Amazon floodplain during the period 26 November to 11 December 2008, 193 iButtons were retrieved in October-November 2009 and 81 (42%) of those were readable. Flooded and non-flooded phases were clearly identifiable for each iButton by the reduced amplitude of the daily temperature wave when flooded. Based on 6 calibration iButtons attached to staff gauges in Mamirauá Lake, the date of flooding could be estimated within ± 3 days. We conclude that Thermochron iButtons provide a relatively low-cost means of acquiring information on inundation period, which can be used for validating satellite-based mapping or hydrologic models. However, because of the harsh conditions occurring on deeply inundated floodplains, improved waterproofing containers are necessary to ensure iButton performance. Further calibration experiments are recommended in order to refine metrics for estimating the flooding date with greater precision.