Background: Food prices influence food choices. Purchasing foods with higher nutritional quality for their price may help improve the diet quality of socioeconomically disadvantaged individuals. Objective: This study aimed to describe the co-construction and evaluation of the Opticourses prevention program, which promotes healthy eating among participants in socioeconomically deprived situations by improving the nutritional quality of their household food purchases with no additional cost. Methods: Individuals were recruited in poor districts of Marseille, France. The intervention and evaluation tools and protocols were co-constructed with 96 individuals. Then, 93 adults willing to participate in a standardized intervention comprising 5 participative workshops on diet and budget were enrolled. Impact on food purchases was estimated with experimental economics: 2-d experimental food purchase intents were observed at baseline and endline for workshop participants (WPs, n = 35) and controls (n = 23), with the use of monetary incentives to limit social-desirability bias. Changes in food and nutrient content and energy cost (expressed in €/2000 kcal) of experimental purchases were assessed. Results: The co-constructed participative workshops included playful activities around food purchase practices and the nutritional quality, taste, and price of foods. Experimental purchases contained a large amount of energy at baseline for both WPs and controls (5114 and 4523 kcal ⋅ d−1 ⋅ person−1, respectively). For WPs only, the mean energy content decreased between baseline and endline (−1729 kcal ⋅ d−1 ⋅ person−1; P < 0.01; medium effect size: Cohen's d = 0.5), and the percentage of energy from free sugars and from foods high in fat, sugar, and salt also decreased (both P < 0.05 and medium effect sizes), whereas energy cost remained unchanged. No significant changes between baseline and endline were observed for the controls. Conclusions: After the intervention, the energy content of participants' experimental purchases was closer to their needs, suggesting that the workshops helped them plan and rationalize their food purchases better. The nutritional quality of the experimental purchases increased but energy cost did not, showing that the co-constructed Opticourses prevention program can favorably change food purchasing behaviors of socioeconomically disadvantaged individuals with no additional cost. This trial was registered at clinicaltrials.gov as NCT02383875.