Lower income is associated with greater stress, and stress has been shown to undermine treatment engagement and weight loss outcomes in face-to-face interventions. The present study examined whether lower income predicts treatment engagement and weight loss outcomes during an online behavioral weight loss (BWL) intervention. A total of N = 260 participants (79.2% female; 50.7 ± 11.9 years of age; 60.3% non-Hispanic White) were enrolled into an online BWL program and had their income, stress, program engagement, and weight measured. Results showed that stress fully mediated the effect of income on engagement in the online program. Further, lower income predicted poorer weight loss outcomes, and this effect was partially mediated by perceived stress. This is the first study to demonstrate that lower income is associated with poorer engagement and weight loss outcomes in an online weight loss program, underscoring the need for systems-level stress reduction programs and individual level stress management tools, particularly for individuals with lower income.