As the most widely used investment mode of transportation infrastructure in China, PPP has been regarded as an effective institutional tool by the Chinese government for the construction of efficient and sustainable transportation infrastructure, promoting high-quality economic development. In order to evaluate the economic sustainability of PPP, this paper innovatively takes the low-carbon economy as the research perspective and uses 276 cities of China from 2006 to 2018 as samples to comprehensively investigate the impact of transportation PPP investment on the low-carbon economy from the diversified dimensions of the overall effect, spatial effect, mediating effect, heterogeneous effect, lagged effect, and PPP project-level characteristics. The results show that: (1) transportation PPP investment markedly promotes low-carbon economic development in China, which is manifested as the dual improvement effects of benefits and efficiency; (2) transportation PPP investment has a positive spatial spillover effect that can promote the low-carbon economic development of neighboring cities; (3) transportation PPP investment can promote low-carbon economic development by promoting the upgrading of industrial structures, and furthermore has a spatial transmission mechanism; (4) impacts of transportation PPP investment in different subsectors are significantly heterogeneous; (5) transportation PPP investment has a continuous and stable impact on promoting the low-carbon economy in China; (6) the participation of state-owned enterprises and listed companies in PPP projects has a significant positive impact, and state-owned capital is more effective in projects with relatively large scale. All these provide reliable evidence for the far-reaching significance and practical value of transportation PPP investment on sustainable low-carbon economic development.