Dietary restriction (DR) is known to have a potent and conserved longevity effect, yet its underlying molecular mechanisms remain elusive. DR modulates signaling pathways in response to nutrient status, a process that also regulates animal development. Here, we show that the suppression of Wnt signaling, a key pathway controlling development, is required for DR‐induced longevity in Caenorhabditis elegans. We find that DR induces the expression of mir‐235, which inhibits cwn‐1/WNT4 expression by binding to the 3′‐UTR. The “switch‐on” of mir‐235 by DR occurs at the onset of adulthood, thereby minimizing potential disruptions in development. Our results therefore implicate that DR controls the adult lifespan by using a temporal microRNA switch to modulate Wnt signaling.