Cities and companies have great potential to reduce pressures on Earth system boundaries. Science-based target setting has emerged as a powerful vehicle, but its uptake is still limited. Moreover, cities and companies develop their targets separately even though many large cities and companies colocate. Focusing on the top emitting 200 cities and 500 companies, here we analyse the current state and potential of adopting science-based targets for climate. Of these key actors, 110 cities and 22 companies with science-based targets can eliminate up to 3.41 GtCO2eq of annual emissions. Further, we argue that if cities and companies keep abreast of ambitious target levels, then this reduction potential can increase up to 5.70 GtCO2eq with a factor of 1.7. We provide an illustrative roadmap with entry points and discuss the implications. Our findings elucidate a previously untapped potential for catalysing city-company collaborations to accelerate transformations for operating within Earth system boundaries.