SummaryCirculating tumour DNA (ctDNA) allows genotyping and minimal residual disease (MRD) detection in lymphomas. Using a next‐generation sequencing (NGS) approach (EuroClonality‐NDC), we evaluated the clinical and prognostic value of ctDNA in a series of R‐CHOP‐treated diffuse large B‐cell lymphoma (DLBCL) patients at baseline (n = 68) and after two cycles (n = 59), monitored by metabolic imaging (positron emission tomography combined with computed tomography [PET/CT]). A molecular marker was identified in 61/68 (90%) ctDNA samples at diagnosis. Pretreatment high ctDNA levels significantly correlated with elevated lactate dehydrogenase, advanced stage, high‐risk International Prognostic Index and a trend to shorter 2‐year progression‐free survival (PFS). Valuable NGS data after two cycles of treatment were obtained in 44 cases, and 38 achieved major molecular response (MMR; 2.5‐log drop in ctDNA). PFS curves displayed statistically significant differences among those achieving MMR versus those not achieving MMR (2‐year PFS of 76% vs. 0%, p < 0.001). Similarly, more than 66% reduction in ΔSUVmax by PET/CT identified two subgroups with different prognosis (2‐year PFS of 83% vs. 38%; p < 0.001). Combining both approaches MMR and ΔSUVmax reduction, a better stratification was observed (2‐year PFS of 84% vs. 17% vs. 0%, p < 0.001). EuroClonality‐NDC panel allows the detection of a molecular marker in the ctDNA in 90% of DLBCL. ctDNA reduction at two cycles and its combination with interim PET results improve patient prognosis stratification.