The aim of this study was to evaluate the impact of the Dietary Approaches to Stop Hypertension (DASH) diet on glycaemic control and consumption of processed (PF) and ultraprocessed (UPF) foods in pregnant women with pre-gestational diabetes mellitus (PGDM). This is a randomised, controlled, single-blind clinical trial with forty-nine adult women with PGDM, followed at a public maternity hospital in Rio de Janeiro, Brazil. The control group (CG) received a standard diet consisting of 45–55 % of the total energy intake of carbohydrates, 15–20 % of proteins and 25–30 % of lipids. The DASH group (DG) received an adapted DASH diet, which did not differ from the standard diet in the percentage of macronutrients, but had higher contents of fibre, unsaturated fats and minerals such as Ca, Mg and K; and lower contents of Na and saturated fats than the standard diet. In the analysis by protocol, the DG presented a higher incidence of glycaemic control after 12 weeks of intervention (57·1 v. 8·3 %, P = 0·01, moderate effect size) and a lower mean consumption of UPF (−9·9 %, P = 0·01) compared with the CG. There was no statistically significant difference in fasting and postprandial blood glucose concentrations, or in the consumption of PF between the groups (P > 0·05). The DASH diet may be a strategy for glycaemic control in pregnant women with PGDM, favouring the adoption of a nutritionally adequate diet with lower consumption of UPF. Further studies are needed to investigate the effect of the DASH diet on glycaemic profile, and maternal and perinatal outcomes in women with PGDM.