To provide the first estimate of the cost-effectiveness of financial incentive for breastfeeding intervention compared with usual care. Within-cluster ('ward'-level) randomised controlled trial cost-effectiveness analysis (trial registration number ISRCTN44898617). Five local authority districts in the North of England. 5398 mother-infant dyads (intervention arm), 4612 mother-infant dyads (control arm). Offering a financial incentive (over a 6-month period) on breast feeding to women living in areas with low breastfeeding prevalence (<40% at 6-8 weeks). Babies breast fed (receiving breastmilk) at 6-8 weeks, and cost per additional baby breast fed. Costs were compared with differences in area-level data on babies' breast fed in order to estimate a cost per additional baby breast fed and the quality-adjusted life year (QALY) gains required over the lifetime of babies to justify intervention cost. In the trial, the total cost of providing the intervention in 46 wards was £462 600, with an average cost per ward of £9989 and per baby of £91. At follow-up, area-level breastfeeding prevalence at 6-8 weeks was 31.7% (95% CI 29.4 to 34.0) in control areas and 37.9% (95% CI 35.0 to 40.8) in intervention areas. The adjusted difference between intervention and control was 5.7 percentage points (95% CI 2.7 to 8.6; p<0.001), resulting in 10 (95% CI 6 to 14) more additional babies breast fed in the intervention wards (39 vs 29). The cost per additional baby breast fed at 6-8 weeks was £974. At a cost per QALY threshold of £20 000 (recommended in England), an additional breastfed baby would need to show a QALY gain of 0.05 over their lifetime to justify the intervention cost. If decision makers are willing to pay £974 (or more) per additional baby breast fed at a QALY gain of 0.05, then this intervention could be cost-effective. Results were robust to sensitivity analyses. This study provides information to help inform public health guidance on breast feeding. To make the economic case unequivocal, evidence on the varied and long-term health benefits of breast feeding to both the baby and mother and the effectiveness of financial incentives for breastfeeding beyond 6-8 weeks is required. © Author(s) (or their employer(s)) 2020. Re-use permitted under CC BY. Published by BMJ.