The school health service is an important service working with health preventive measures for youth. However, challenges exist, for example in terms of resources. Chatbots represent new opportunities for strengthening the school health service. This thesis is an exploratory study of designing a chatbot to complement the preventive work of school nurses in the school health service, with particular regard for youths’ mental health issues. Through a design research approach utilizing a user-center design process I have explored how a chatbot can offer support for youths’ mental health issues. I have collected user and domain insight through interviews and focus group and conducted concept development through sketches. A prototype has been designed and evaluated through user tests. The user and domain insight indicate that youths’ mental health issues are multifaceted and that complementary needs arise in response to this, suggesting that a service intended to complement the school health service will benefit from providing a range of support. I have developed concepts and design principles for a chatbot intended to provide support in response to youths’ mental health issues, with particular regard for informational, relational and processual support, as well as guiding youths to support by human professionals when need be. The prototype in this study represents something new in comparison to related work, as it explores how to offer a wider range of support in a single chatbot. This is a step in the direction of establishing needed knowledge about how different aspects of a chatbot may be designed in the context of preventive mental health support for youths.