Circadian clocks are near-ubiquitous molecular oscillators that coordinate biochemical, physiological, and behavioral processes with environmental cues, such as dawn and dusk. Circadian timing mechanisms are thought to have arisen multiple times throughout the evolution of eukaryotes but share a similar overall structure consisting of interlocking transcriptional and posttranslational feedback loops. Recent work in both plants and animals has also linked modification of histones to circadian clock function. Now, using data from published microarray experiments, we have identified a histone demethylase, jumonji domain containing 5 (JMJD5), as a previously undescribed participant in both the human andArabidopsiscircadian systems.Arabidopsis JMJD5is coregulated with evening-phased clock components and positively affects expression of clock genes expressed at dawn. We found that bothArabidopsis jmjd5mutant seedlings and mammalian cell cultures deficient for the human ortholog of this gene have similar fast-running circadian oscillations compared with WT. Remarkably, both theArabidopsisand humanJMJD5orthologs retain sufficient commonality to rescue the circadian phenotype of the reciprocal system. Thus, JMJD5 plays an interchangeable role in the timing mechanisms of plants and animals despite their highly divergent evolutionary paths.