Circadian clocks are endogenous timers adjusting behaviour and physiology with the solar day(1). Synchronized circadian clocks improve fitness(2) and are crucial for our physical and mental well-being(3). Visual and non-visual photoreceptors are responsible for synchronizing circadian clocks to light(4,5), but clock-resetting is also achieved by alternating day and night temperatures with only 2-4 degrees C difference(6-8). This temperature sensitivity is remarkable considering that the circadian clock period (similar to 24 h) is largely independent of surrounding ambient temperatures(1,8). Here we show that Drosophila Ionotropic Receptor 25a (IR25a) is required for behavioural synchronization to low-amplitude temperature cycles. This channel is expressed in sensory neurons of internal stretch receptors previously implicated in temperature synchronization of the circadian clock(9). IR25a is required for temperature-synchronized clock protein oscillations in subsets of central clock neurons. Extracellular leg nerve recordings reveal temperature-and IR25a-dependent sensory responses, and IR25a misexpression confers temperature-dependent firing of heterologous neurons. We propose that IR25a is part of an input pathway to the circadian clock that detects small temperature differences. This pathway operates in the absence of known 'hot' and 'cold' sensors in the Drosophila antenna(10,11), revealing the existence of novel periphery-to-brain temperature signalling channels.