Background Numerous studies describe the popularity and usefulness of parenting programs. In particular, parenting programs are generally viewed as effective for supporting parents’ mental well-being during key transition periods. However, the evidence base for fathers is limited owing to their lack of involvement in parenting programs and scarcity of tailored support. Objective This paper aimed to describe the co-design process for a universal digital intervention for fathers (fatherli) and the outline of a logic model with its expected outcomes. Methods Following established guidelines for co-designing and developing complex interventions, we conducted a nonsystematic review of the available literature to gather key information, developed market surveys to assess fathers’ needs and interests, consulted with key stakeholders to obtain expert opinions, and engaged in a rapid iterative prototyping process with app developers. Each step was summarized, and the information was collated and integrated to inform a logic model and the features of the resulting intervention. Results The steps in the co-design process confirmed a need for and interest in a digital intervention for fathers. In response to this finding, fatherli was developed, consisting of 5 key features: a discussion forum for anyone to post information about various topics (the forum), a socializing platform for fathers to create and engage with others in small groups about topics or points of shared interest (dad hub), a tool for fathers to find other fathers with shared interests or within the same geographic location (dad finder), a resource for fathers to access up-to-date information about topics that interest them (dad wiki), and a portal to book sessions with coaches who specialize in different topics (dad coaching space). The evidence-based logic model proposes that if fatherli is successfully implemented, important outcomes such as increased parental efficacy and mental health help-seeking behaviors may be observed. Conclusions We documented the co-design and development process of fatherli, which confirmed that it is possible to use input from end users and experts, integrated with theory and research evidence, to create suitable digital well-being interventions for fathers. In general, the key findings suggest that an app that facilitates connection, communication, and psychoeducation may appeal to fathers. Further studies will now focus on acceptability, feasibility, and effectiveness. Feedback gathered during pilot-testing will inform any further developments in the app to increase its applicability to fathers and its usability.

UNSTRUCTURED Introduction Parenting programmes are generally viewed as effective for supporting parents’ mental wellbeing during key transition periods. However, the evidence-base for fathers is limited due to their lack of involvement in parenting programmes and scarcity of tailored support. This article describes the co-design process for a universal digital intervention for fathers and a logic model with its expected outcomes. Methods Following established guidelines for co-designing and developing complex interventions we conducted 1) a non-systematic review of the available literature to gather key information, 2) market surveys to assess fathers’ needs and interests, 3) consultations with key stakeholders to obtain expert opinions and 4) engaged in a rapid iterative prototyping process with app developers. Each step was summarised, and the information collated and integrated to inform a logic model and the features of the resulting intervention. Results The steps in the co-design process confirmed a need for, and interest in, a digital intervention for fathers. In response to this, fatherli was developed consisting of five key features 1) a discussion forum for anyone to post information about various topics (The Forum), 2) a socialising platform for fathers to create and engage with others in small groups on topics or points of shared interest (Dad Hub), 3) a tool for fathers to find other fathers with shared interests or within the same geography (Dad Finder), 4), a resource for dads to access up-to-date information on topics that interest them (Dad Wiki and, 5) the coaching space, a portal to book sessions with coaches that specialise on different topics. The evidence-based logic model proposes that if fatherli is successfully implemented, important outcomes such as increased parental efficacy and mental health help-seeking behaviours may be observed. Conclusion We documented the codesign and development process of fatherli which confirmed that it is possible to use input from end users and experts, integrated with theory and research evidence to create suitable digital wellbeing interventions for fathers. Further research will now focus on acceptability, feasibility, and effectiveness.