Malnutrition in all its forms broadly includes undernutrition (e.g. underweight and lack of certain key nutrients) and overnutrition (e.g. obesity and related chronic non-communicable diseases, such as type 2 diabetes). Malnutrition in all its forms is the largest cause of disease and premature death globally and in South-East Asia (SEA). Its health impacts are twice as high as that of tobacco use and high blood pressure. This public health challenge is also associated with heavy social and economic burdens. Different forms of malnutrition share common causes, especially within food systems that are promoting these various forms of malnutrition, which can co-occur in the same individual, household or country at the same time. Locally relevant solutions will need to be generated in relation to people's eating habits and food cultures (e.g. the local food systems) as well as general living environments. Therefore, the seemingly different nutrition problems can be improved through common solutions targeting the wider systems of people's daily lives. Current approaches to interventions are not reducing undernutrition fast enough and have failed to control the rise of obesity and related chronic diseases. These problems have rarely been considered and managed together in an integrated way. To effectively combat this largely preventable public health challenge, we need a new way of working that recognises the connections between different forms and causes of malnutrition within a wider context and creates double-duty actions to address them together. We aim to develop public health interventions in Chinese and SEA cities that are: 1) jointly enacted by multiple sectors, 2) aimed to improve multiple forms of malnutrition, and 3) expected to benefit everyone living in these cities. We will do this collectively with local policy makers from different departments and sectors as well as community representatives (e.g. those who have delivered or received existing services or programmes aimed to improve nutrition status) over three project phases using a contemporary research method called the Group Model Building (GMB). GMB is a useful tool to develop a shared understanding of complex, inter-related issues and to facilitate coordinated actions among different people. It has been successfully used by members of this research team for developing systemic-level obesity interventions in developed countries. The proposed project (phase 1) will provide a strong foundation for subsequent project phases by: 1) co-developing systems interventions through GMB and forming intervention delivery Action Groups with local decision makers and community representatives in an Asian city; and gaining practical insights into this new intervention development method for application in other Asian cities, 2) identifying strategies to recruit SEA cities to join the project, and 3) providing information needed to support the development of an internationally comparable and sustainable tools to monitor and assess impacts of developed interventions. In the subsequent, phase 2 project, we will 1) support the Action Groups to deliver the interventions developed in phase 1, and measure early impacts of the interventions in the first Asian city using monitoring systems informed by phase 1; and 2) recruit SEA cities, develop interventions in these cities using the GMB process and form Action Groups. This will be the first research project to use GMB to develop malnutrition interventions in developing, Asian countries. Our findings will importantly advance the work on the Decade of Action on Nutrition towards achieving the United Nations' Sustainable Development Goals and contribute to method development and impacts of this new way of working on public health promotion globally.