Small carbocycles are structurally rigid and possess high intrinsic energy due to their ring strain. These features lead to broad applications but also create challenges for their construction. We report the engineering of hemeproteins that catalyze the formation of chiral bicyclobutanes, one of the most strained four-membered systems, via successive carbene addition to unsaturated carbon-carbon bonds. Enzymes that produce cyclopropenes, putative intermediates to the bicyclobutanes, were also identified. These genetically encoded proteins are readily optimized by directed evolution, function in Escherichia coli, and act on structurally diverse substrates with high efficiency and selectivity, providing an effective route to many chiral strained structures. This biotransformation is easily performed at preparative scale, and the resulting strained carbocycles can be derivatized, opening myriad potential applications.