Does a carbon tax stimulate low-carbon innovation in the transport sector? We exploit an environmental tax reform in Sweden that introduced both a carbon and a value-added tax on transport fuels. Synthetic control estimates consistently find that the tax reform caused large and significant increases in low-carbon patents. We provide suggestive evidence that the carbon tax drives the effect while fuel price changes with comparable magnitude did not significantly affect low-carbon innovation. Our findings suggest that salient carbon prices may foster low-carbon innovation more effectively than fuel price elasticity estimates suggest.