Constructed wetlands (CWs) for wastewater treatment may be part of the answer to the urgent need for a change in the approach to wastewater treatment in developing countries. Although Thailand has several CWs, there have been no studies of their sustainability. To remedy this, the sustainability of three promising CWs in very different settings was assessed. These were located at Koh Phi Phi, a world-renowned international tourist and holiday resort; Sakon Nakhon, a northeastern provincial capital; and Ban Pru Teaw, a small post-tsunami village on the Andaman coast. Key stakeholder interviews, questionnaires and household interview surveys, together with existing data and on-site measurements of the key pollutant content of wastewater were used to evaluate the systems. Results show that major management and treatment problems have emerged in the projects at Koh Phi Phi and Ban Pru Teaw due to the lack of post-construction personnel development and maintenance; but on the other hand, Sakon Nakhon is the first CW in Thailand to obtain ISO 9001 certification. The results reveal the importance of the socio-cultural dimension of sustainability; public perception, awareness and knowledge, local expertise and clear roles for institutions could explain the differences in sustainability of the CWs. The environmental benefits and the low operation and maintenance costs are also important for sustainability, by justifying the system and avoiding user payments.