Background Influenza immunisation during pregnancy is recommended but not widely implemented in some low-income regions. We assessed the safety and efficacy in mothers and infants of year-round maternal influenza immunisation in Nepal, where influenza viruses circulate throughout the year. Methods In this phase 4, randomised, placebo-controlled trial, we enrolled two consecutive sequential annual cohorts of pregnant women from the Sarlahi district in southern Nepal. We randomised mothers 1:1 to receive seasonally recommended trivalent inactivated influenza vaccine or saline placebo in blocks of eight, stratified by gestational age at enrolment (17–25 weeks vs 26–34 weeks). Women were eligible if they were married, 15–40 years of age, 17–34 weeks' gestation at enrolment, and had not previously received any influenza vaccine that season. We collected serum samples before and after immunisation, and cord blood from a subset of women and infants. Staff masked to allocation made home visits every week from enrolment to 6 months after delivery. Midnasal swabs for respiratory virus PCR testing were collected during maternal acute febrile respiratory infections, and from infants with any respiratory symptom. We assessed vaccine immunogenicity, safety, and three primary outcomes: the incidence of maternal influenza-like illness in pregnancy and 0–180 days postpartum, the incidence of low birthweight (<2500 g), and the incidence of laboratory-confirmed infant influenza disease from 0 to 180 days. This trial is registered with ClinicalTrials.gov, number NCT01034254. Findings From April 25, 2011, to Sept 9, 2013, we enrolled 3693 women in two cohorts of 2090 (1041 assigned to placebo and 1049 to vaccine) and 1603 (805 assigned to placebo and 798 to vaccine), with 3646 liveborn infants (cohort 1, 999 in placebo group and 1010 in vaccine group; cohort 2, 805 in placebo group and 798 in vaccine group). Immunisation reduced maternal febrile influenza-like illness with an overall efficacy of 19% (95% CI 1 to 34) in the combined cohorts; 9% efficacy (−16 to 29) in the first cohort, and 36% efficacy (9 to 55) in the second cohort. For laboratory-confirmed influenza infections in infants aged 0–6 months, immunisation had an overall efficacy for the combined cohorts of 30% (95% CI 5 to 48); in the first cohort, the efficacy was 16% (−19 to 41), and in the second cohort it was 60% (26 to 88). Maternal immunisation reduced the rates of low birthweight by 15% (95% CI 3–25) in both cohorts combined. The rate of small for gestational age infants was not modified by immunisation. The number of adverse events was similar regardless of immunisation status. Miscarriage occurred in three (0·2%) participants in the placebo group versus five (0·3%) in the vaccine group, stillbirth occurred in 31 (1·7%) versus 33 (1·8%), and congenital defects occurred in 18 (1·0%) versus 20 (1·1%). Five women died in the placebo group and three died in the vaccine group. The number of infant deaths at age 0–6 months was similar in each group (50 in the placebo group and 61 in the vaccine group). No serious adverse events were associated with receipt of immunisation. Interpretation Year-round maternal influenza immunisation significantly reduced maternal influenza-like illness, influenza in infants, and low birthweight over the entire course of the study, indicating the strategy could be useful in subtropical regions. Funding Bill & Melinda Gates Foundation.