Engineering light-sensitive protein regulators has been a tremendous multidisciplinary challenge. Optogenetic regulators of MAPKs， central nodes of cellular regulation， have not previously been described. Here we present OptoJNKi， a light-regulated JNK inhibitor based on the AsLOV2 light-sensor domain using the ubiquitous FMN chromophore. OptoJNKi gene-transfer allows optogenetic applications， whereas protein delivery allows optopharmacology. Development of OptoJNKi suggests a design principle for other optically regulated inhibitors. From this， we generate Optop38i， which inhibits p38MAPK in intact illuminated cells. Neurons are known for interpreting temporally-encoded inputs via interplay between ion channels， membrane potential and intracellular calcium. However， the consequences of temporal variation of JNK-regulating trophic inputs， potentially resulting from synaptic activity and reversible cellular protrusions， on downstream targets are unknown. Using OptoJNKi， we reveal maximal regulation of c-Jun transactivation can occur at unexpectedly slow periodicities of inhibition depending on the inhibitor's subcellular location. This provides evidence for resonance in metazoan JNK-signalling circuits.