Summary Weekly ixazomib with lenalidomide‐dexamethasone (Rd) is feasible and has shown activity in newly diagnosed multiple myeloma (NDMM) patients. This phase 1/2 study (NCT01383928) evaluated the recommended phase 2 dose (RP2D), pharmacokinetics, safety and efficacy of twice‐weekly ixazomib plus Rd in NDMM; 64 patients were enrolled across both phases. Patients received twice‐weekly oral ixazomib 3·0 or 3·7 mg plus lenalidomide 25 mg and dexamethasone 20 mg (10 mg in cycles 9–16) for up to sixteen 21‐day cycles, followed by maintenance with twice‐weekly ixazomib alone. No dose‐limiting toxicities were reported in cycle 1; the RP2D was 3·0 mg based on overall tolerability across multiple cycles. In 62 evaluable patients, the confirmed overall response rate was 94% (68% ≥very good partial response; 24% complete response). Median progression‐free survival was 24·9 months. Responses (median duration 36·9 months for patients receiving the RP2D) deepened during treatment. Grade 3 drug‐related adverse events (AEs) occurred in 64% of patients, including: rash, 13%; peripheral neuropathy, 8%; hyperglycaemia, 8%. There were no grade 4 drug‐related AEs. Thirteen patients discontinued due to AEs. Twice‐weekly ixazomib‐Rd offers substantial activity with promising long‐term outcomes in NDMM patients but may be associated with greater toxicity compared with weekly ixazomib‐Rd in this setting.