Cyanobacteria can be utilized as a platform for direct phototrophic conversion of CO to produce several types of carbon-neutral biofuels. One promising compound to be produced photobiologically in cyanobacteria is isobutene. As a volatile compound, isobutene will quickly escape the cells without building up to toxic levels in growth medium or get caught in the membranes. Unlike liquid biofuels, gaseous isobutene may be collected from the headspace and thus avoid the costly extraction of a chemical from culture medium or from cells. Here we investigate a putative synthetic pathway for isobutene production suitable for a photoautotrophic host. First, we expressed α-ketoisocaproate dioxygenase from (KICD) in . We discovered isobutene formation with the purified KICD with the rate of 104.6 ​± ​9 ​ng (mg protein) min using α-ketoisocaproate as a substrate. We further demonstrate isobutene production in the cyanobacterium sp. PCC 6803 by introducing the KICD enzyme. strain heterologously expressing the KICD produced 91 ​ng ​l OD ​h. Thus, we demonstrate a novel sustainable platform for cyanobacterial production of an important building block chemical, isobutene. These results indicate that KICD can be used to further optimize the synthetic isobutene pathway by protein and metabolic engineering efforts.