Higher monetary diet cost is associated with healthier food choices and better weight management. How changes in diet cost affect changes in diet quality and weight remains unknown. The aim of this study was to assess the impact of changes in individual monetary diet cost on changes in diet quality, measured by the modified Mediterranean diet score recommendations (MDS-rec) and by energy density (ED), as well as changes in weight and BMI. We conducted a prospective, population-based study of 2181 male and female Spaniards aged between 25 and 74 years, who were followed up to the 2009-2010 academic year. We measured weight and height and recorded dietary data using a validated FFQ. Average food cost was calculated from official Spanish government data. We fitted multivariate linear and logistic regression models. The average daily diet cost increased from 3·68(SD0.0·89) /8·36 MJ to 4·97(SD1·16) /8·36 MJ during the study period. This increase was significantly associated with improvement in diet quality (Δ ED and Δ MDS-rec; P<0·0001). Each 1 increase in monetary diet cost per 8·36 MJ was associated with a decrease of 0·3 kg in body weight (P=0·02) and 0·1 kg/m2 in BMI (P=0·04). These associations were attenuated after adjusting for changes in diet quality indicators. An improvement in diet quality and better weight management were both associated with an increase in diet cost; this could be considered in food policy decisions.