Open top chambers (OTCs) are a commonly used passive warming technique in experimental warming studies. OTCs have been shown to be effective in multiple types of terrestrial systems, but their utility in wetland environments remains uncertain. The objective of this work was to evaluate the effectiveness of using OTCs to warm a temperate salt marsh across diurnal and seasonal cycles. We found that OTCs are effective at warming air temperatures on the marsh, with average air temperatures 1.6 ± 0.007 °C and 1.1 ± 0.006 °C warmer within the high and low marsh, respectively over a 16-month period. In contrast, OTCs were ineffective at warming sediments, especially during the day. In fact, sediment temperatures within the OTC were cooler during the day relative to ambient conditions. Such daytime warming of air, but cooling of sediments relative to ambient conditions resulted in a significant decoupling of above and belowground temperatures in the marsh (r = -0.99 and -0.82 on low and high marsh, respectively). Our data indicate that shading by OTCs was responsible for the daytime sediment cooling relative to ambient conditions during most of the year, as incoming solar radiation was reduced by 30% within OTCs. Wet sediments require more energy to heat than the air due to their higher specific heat capacity. Thus, reductions in radiation by OTCs prevented effective warming of sediments, but still allowed for the warming of air. In turn, we conclude that OTCs are not an effective method to experimentally warm tidal marsh sediments.