In Tanzania, street traders face the challenge of limited markets caused by employing weak marketing and promotion strategies. This study developed a mobile application to solve the problem addressed using participatory design. Qualitative data were collected using focus group discussions and brainstorming from 80 respondents involving both street traders and customers in different workshops and meetings. Data were used for the design and development of the Machinga application. Furthermore, quantitative data for application evaluation were collected from 96 respondents using questionnaires. In addition, 20 interviews were conducted to validate the evaluation results. Thematic and descriptive analysis were performed for both qualitative and quantitative data. The results show that the mobile application has prospective features which solve the problem of limited markets in the street trading community. The application is perceived positively by end-users because of embracing their prior requirements and meeting the evaluation criteria of usefulness, ease-of-use, learnability, and user satisfaction. The study recommends further training of users to enable the application to attain its multiplier effect on the vast population. This study confirms the relevance of participatory design in ICT4D projects for informal workers as it allowed the involvement of end-users and reflected their voices in terms of the technology they desire.