Numerous observational studies have investigated associations between diet indices and health outcomes. Our aim was to systematically synthesise data that was previously summarised separately for each diet index in one umbrella review of all diet indices with sufficient evidence gained in systematic reviews and to assess the quality and strength of evidence for selected health outcomes. The MEDLINE, EMBASE and Scopus databases were systematically searched following the PRISMA guidelines through October 2021 for systematic reviews of observational studies investigating associations between adherence to diet indices and selected health outcomes (all-cause mortality, cardiovascular disease incidence or mortality, type 2 diabetes incidence or mortality). Methodological quality and quality of evidence were assessed using the AMSTAR 2 and NutriGrade tools. The inclusion criteria were met by seven systematic reviews, entirely based on prospective cohort studies and reviewing five different diet indices – Alternate Healthy Eating Index (AHEI), Dietary Approaches to Stop Hypertension (DASH), Dietary Inflammatory Index (DII), Healthy Eating Index (HEI), Mediterranean diet (MedDiet). All seven included systematic reviews showed that greater adherence to these diet indices reduces the risks of all-cause mortality, cardiovascular disease (CVD) incidence and mortality and type 2 diabetes incidence. Moderate meta-evidence was presented for AHEI and DASH for all outcomes, also for DII for all-cause mortality, CVD mortality and incidence, MedDiet for all-cause mortality and for HEI for CVD incidence and mortality. Our umbrella review provides further evidence for AHEI, DASH, DII, and HEI diet indices to be used as predictors of selected health outcomes.