Eukaryotic mRNA decapping proteins are essential for normal turnover of mRNA. Yet, the mechanism of bulk mRNA turnover during stress responses and its importance to stress tolerance are poorly understood. Here, we showed that dehydration stress activated MPK6 to phosphorylate serine 237 of Arabidopsis DCP1 and phospho-DCP1 preferentially associated with DCP5 to promote mRNA decapping in vivo. This process was essential for stress adaption as dcp5-1 and DCP1-S237A plants were hypersensitive to stress compared with wild-type (WT) plants. Microarray analysis revealed that dehydration-induced expression of many stress responsive genes was compromised in dcp5-1, whereas a subset of transcripts was over-represented in this mutant. Further analysis revealed that this subset of transcripts was likely the direct targets of stress-triggered mRNA decapping in WT. Our results suggest that mRNA decapping through MPK6-DCP1-DCP5 pathway serves as a rapid response to dehydration stress in Arabidopsis.