Introduction: Dietary diversity is a global challenge in complementary feeding. Despite more women joining the workforce in developing countries, there are limited studies on the beliefs of working mothers and their experiences in relation to the provision of dietary diversity as recommended by the World Health Organization. Methods: This qualitative study explored the behavioural, normative and control beliefs of working mothers on dietary diversity practices, based on the Theory of Planned Behaviour (TPB). A total of 25 mothers of different occupational levels were recruited from workplaces in Jakarta. Results: Working mothers at the lower occupational levels showed a lack of understanding of the importance of dietary diversity and reported poor practices. These included the late introduction of animal protein as a food source, and few types of feeding instant foods. Due to their limited knowledge of nutrition, these working mothers tended to accept poor dietary diversity practices as normal. Conclusion: Working mothers at the lower occupational levels practised poor dietary diversity owing to work-related factors. Efforts should be undertaken to provide correct nutritional information related to complementary feeding at workplaces, especially to working mothers in the unskilled occupations.