Transitioning to plant-dominant dishes in the growing meal kit delivery market could significantly reduce the greenhouse gas emission footprint of the sector. However, many consumers choose meat-based options without considering plant-based alternatives when ordering food online. This scenario-based survey experiment with 669 consumers tested whether an environmental message between lunch and dinner would increase stated plant-based dinner choices. The findings show that the environmental message significantly increased plant-based dinner selections through two pathways. For consumers who chose a plant-based lunch, the message reinforced their environmental identity, leading to more plant-based dinner choices. The message induced cognitive dissonance for those who chose a meat-based lunch, increasing the likelihood of choosing plant-based dinner options. These results indicate that an environmental message can effectively influence meal choices through different mechanisms, offering a cost-effective strategy for practitioners and policymakers to promote environmentally sustainable dining habits in the online meal kit delivery sector.