A transboundary problem is an environmental problem that is transboundary in scale. In other words, it is an environmental problem originating in, or contributed by, one country and affecting (or impacting) another.

For example, in the case of eutrophication in the Dnipro River Basin (a transboundary problem affecting many aquatic systems), the nutrients may be emitted predominantly by one country in a region but the effects felt in several countries. The impact may be damage to the natural environment (e.g. algal blooms) and/or damage to human welfare (e.g. health problems).

Likewise, the loss of coastal habitats in the Mediterranean is a transboundary problem. For example, the loss of nesting sites for Loggerhead Turtles in a number of Mediterranean countries (together with accidental capture in fishing gear) has resulted in the rapid decline of this global migratory species.