Background: It is unknown whether a socio-economic difference exists in the association of diet quality with type 2 diabetes incidence, nor how diet influences the socioeconomic inequality in diabetes burden. Methods: In 91,025 participants of the population-based Lifelines Cohort (aged ≥30, no diabetes or cardiovascular diseases at baseline), type 2 diabetes incidence was based on self-report, fasting glucose ≥ 7·0 mmol/l and/or HbA1c ≥ 6·5%. The evidence-based Lifelines Diet Score was calculated with data of a 110-item food frequency questionnaire. Socio-economic status (SES) was defined by educational level. Cox proportional hazards models were adjusted for age, gender, smoking, energy intake, alcohol intake and physical activity. Findings: In 279,796 person-years of follow-up, 1045 diabetes cases were identified. Incidence rate was 5·7, 3·2 and 2·4 cases/1000 person-years in low, middle and high SES, respectively. Diet was associated with greater diabetes risk (HR(95%CI) in Q1 (poor diet quality) vs. Q5 (high diet quality) = 2·11 (1·70–2·62)). SES was a moderator of the association(pINTERACTION = 0·038). HRs for Q1 vs. Q5 were 1·66 (1·22–2·.27) in low, 2·76 (1·86–4·08) in middle and 2·46 (1·53–3·97) in high SES. With population attributable fractions of 14·8%, 40·1% and 37·3%, the expected number of cases/1000 person-years preventable by diet quality improvement was 0·85 in low, 1·28 in middle and 0·90 in high SES. Interpretation: Diet quality improvement can potentially prevent one in three cases of type 2 diabetes, but because of a smaller impact in low SES, it will not narrow the socioeconomic health gap in diabetes burden. Funding: None. Keywords: Socio-economic status, Health inequality, Socio-economic inequality, Diet quality, Type 2 diabetes, Prospective cohort study, Public health