Objective To evaluate the efficacy of financial incentives dependent on continuous smoking abstinence on smoking cessation and birth outcomes among pregnant smokers. Design Single blind, randomised controlled trial. Setting Financial Incentive for Smoking Cessation in Pregnancy (FISCP) trial in 18 maternity wards in France. Participants 460 pregnant smokers aged at least 18 years who smoked ≥5 cigarettes/day or ≥3 roll-your-own cigarettes/day and had a pregnancy gestation of <18 weeks were randomised to a financial incentives group (n=231) or a control group (n=229). Interventions Participants in the financial incentives group received a voucher equivalent to €20 (£17; $23), and further progressively increasing vouchers at each study visit if they remained abstinent. Participants in the control group received no financial incentive for abstinence. All participants received a €20 show-up fee at each of six visits. Main outcome measures The main outcome measure was continuous smoking abstinence from the first post-quit date visit to visit 6, before delivery. Secondary outcomes in the mothers were point prevalence abstinence, time to smoking relapse, withdrawal symptoms, blood pressure, and alcohol and cannabis use in past 30 days. Secondary outcomes in the babies were gestational age at birth, birth characteristics (birth weight, length, head circumference, Apgar score), and a poor neonatal outcome—a composite measure of transfer to the neonatal unit, congenital malformation, convulsions, or perinatal death. Results Mean age was 29 years. In the financial incentives and control groups, respectively, 137 (59%) and 148 (65%) were employed, 163 (71%) and 171 (75%) were in a relationship, and 41 (18%) and 31 (13%) were married. The participants had smoked a median of 60 cigarettes in the past seven days. The continuous abstinence rate was significantly higher in the financial incentives group (16%, 38/231) than control group (7%, 17/229): odds ratio 2.45 (95% confidence interval 1.34 to 4.49), P=0.004). The point prevalence abstinence rate was higher (4.61, 1.41 to 15.01, P=0.011), the median time to relapse was longer (visit 5 (interquartile range 3-6) and visit 4 (3-6), P<0.001)), and craving for tobacco was lower (β=−1.81, 95% confidence interval −3.55 to −0.08, P=0.04) in the financial incentives group than control group. Financial incentives were associated with a 7% reduction in the risk of a poor neonatal outcome: 4 babies (2%) in the financial incentives group and 18 babies (9%) in the control group: mean difference 14 (95% confidence interval 5 to 23), P=0.003. Post hoc analyses suggested that more babies in the financial incentives group had birth weights ≥2500 g than in the control group: unadjusted odds ratio 1.95 (95% confidence interval 0.99 to 3.85), P=0.055; sex adjusted odds ratio 2.05 (1.03 to 4.10), P=0.041; and sex and prematurity adjusted odds ratio 2.06 (0.90 to 4.71), P=0.086. As these are post hoc analyses, the results should be interpreted with caution. Conclusions Financial incentives to reward smoking abstinence compared with no financial incentives were associated with an increased abstinence rate in pregnant smokers. Financial incentives dependent on smoking abstinence could be implemented as a safe and effective intervention to help pregnant smokers quit smoking.