Sustainable transport is transportation sustainable in terms of their social and environmental impacts. Components for evaluating sustainability include the particular vehicles used; the source of energy; and the infrastructure used to accommodate the transport (streets and roads, railways, airways, waterways and canals). Transportation sustainability is largely being measured by transportation system effectiveness and efficiency as well as the environmental and climate impacts of the system. Transport systems have significant impacts on the environment. In 2018, it contributed to around 20% of global CO₂ emissions. Greenhouse gas emissions from transport are increasing at a faster rate than any other energy using sector. A 2023 study published in Journal of Transport Geography found that shared electric bicycle systems reduce urban transport-related carbon emissions by about 108–120 grams per kilometre, particularly in non-central urban areas and when powered by low-carbon electricity sources. Road transport is also a major contributor to local air pollution and smog.

Sustainable transport systems make a positive contribution to the environmental, social and economic sustainability of the communities they serve. Transport systems exist to provide social and economic connections, and people quickly take up the opportunities offered by increased mobility, with poor households benefiting greatly from low carbon transport options. The advantages of increased mobility need to be weighed against the environmental, social and economic costs that transport systems pose. Short-term activity often promotes incremental improvement in fuel efficiency and vehicle emissions controls while long-term goals include migrating transportation from fossil-based energy to other alternatives such as renewable energy and use of other renewable resources. The entire life cycle of transport systems is subject to sustainability measurement and optimization.

The United Nations Environment Programme (UNEP) estimates that each year 2.4 million premature deaths from outdoor air pollution could be avoided. Particularly hazardous for health are emissions of black carbon, a component of particulate matter, which is a known cause of respiratory and carcinogenic diseases and a significant contributor to global climate change. The links between greenhouse gas emissions and particulate matter make low carbon transport an increasingly sustainable investment at local level—both by reducing emission levels and thus mitigating climate change; and by improving public health through better air quality. The term "green mobility" also refers to clean ways of movement or sustainable transport.

The social costs of transport include road crashes, air pollution, physical inactivity, time taken away from the family while commuting and vulnerability to fuel price increases. Many of these negative impacts fall disproportionately on those social groups who are also least likely to own and drive cars. Traffic congestion imposes economic costs by wasting people's time and by slowing the delivery of goods and services. Traditional transport planning aims to improve mobility, especially for vehicles, and may fail to adequately consider wider impacts. But the real purpose of transport is access – to work, education, goods and services, friends and family – and there are proven techniques to improve access while simultaneously reducing environmental and social impacts, and managing traffic congestion. Communities which are successfully improving the sustainability of their transport networks are doing so as part of a wider program of creating more vibrant, livable, sustainable cities.

The term sustainable transport came into use as a logical follow-on from sustainable development, and is used to describe modes of transport, and systems of transport planning, which are consistent with wider concerns of sustainability. There are many definitions of the sustainable transport, and of the related terms sustainable transportation and sustainable mobility. One such definition, from the European Union Council of Ministers of Transport, defines a sustainable transportation system as one that:

Sustainability extends beyond just the operating efficiency and emissions. A life-cycle assessment involves production, use and post-use considerations. A cradle-to-cradle design is more important than a focus on a single factor such as energy efficiency.

Sustainable transport has many social and economic benefits that can accelerate local sustainable development. According to a series of serious reports by the Low Emission Development Strategies Global Partnership (LEDS GP), sustainable transport can help create jobs, improve commuter safety through investment in bicycle lanes, pedestrian pathways and non-pedestrian pathways, make access to employment and social opportunities more affordable and efficient. It also offers a practical opportunity to save people's time and household income as well as government budgets, making investment in sustainable transport a 'win-win' opportunity.

Transport systems are major emitters of greenhouse gases, responsible for 23% of world energy-related GHG emissions in 2004, with about three-quarters coming from road vehicles. Data from 2011 stated that one-third of all greenhouse gases produced are due to transportation. Currently 95% of transport energy comes from petroleum. Energy is consumed in the manufacture as well as the use of vehicles, and is embodied in transport infrastructure including roads, bridges and railways. Motorized transport also releases exhaust fumes that contain particulate matter which is hazardous to human health and a contributor to climate change.