Background It is estimated that 1.5 billion people are infected with soil‐transmitted helminths (STHs) worldwide. Re‐infection occurs rapidly following deworming, and interruption of transmission is unlikely without complementary control efforts such as improvements in water, sanitation, and hygiene (WASH) access and behaviours. Objectives To assess the effectiveness of WASH interventions to prevent STH infection. Search methods We used standard, extensive Cochrane search methods. The latest search date was 19 October 2021. Selection criteria We included interventions to improve WASH access or practices in communities where STHs are endemic. We included randomized controlled trials (RCTs), as well as trials with an external control group where participants (or clusters) were allocated to different interventions using a non‐random method (non‐RCTs). We did not include observational study designs. Our primary outcome was prevalence of any STH infection. Prevalence of individual worms was a secondary outcome, including for Ascaris lumbricoides , Trichuris trichiura , hookworm ( Ancylostoma duodenale or Necator americanus ), or Strongyloides stercoralis . Intensity of infection, measured as a count of eggs per gram of faeces for each species, was another secondary outcome. Data collection and analysis Two review authors independently reviewed titles and abstracts and full‐text records for eligibility, performed data extraction, and assessed risk of bias using the Cochrane risk of bias assessment tool for RCTs and the EPOC tool for non‐RCTs. We used a random‐effects meta‐analysis to pool study estimates. We used Moran’s I² statistic to assess heterogeneity and conducted subgroup analyses to explore sources of heterogeneity. We assessed the certainty of the evidence using the GRADE approach. Main results We included 32 studies (16 RCTs and 16 non‐RCTs) involving a total of 52,944 participants in the review. Twenty‐two studies (14 RCTs (16 estimates) and eight non‐RCTs (11 estimates)) reported on our primary outcome, prevalence of infection with at least one STH species. Twenty‐one studies reported on the prevalence of A lumbricoides (12 RCTs and 9 non‐RCTs); 17 on the prevalence of T trichiura (9 RCTs and 8 non‐RCTs); 18 on the prevalence of hookworm (10 RCTs and 8 non‐RCTs); and one on the prevalence of S stercoralis (1 non‐RCT). Sixteen studies measured the intensity of infection for an individual STH type. Ten RCTs and five non‐RCTs reported on the intensity of infection of A lumbricoides ; eight RCTs and five non‐RCTs measured the intensity of infection of T trichiura ; an...