IMPORTANCE The effects of intensive care unit (ICU) visiting hours remain uncertain. OBJECTIVE To determine whether a flexible family visitation policy in the ICU reduces the incidence of delirium. DESIGN, SETTING AND PARTICIPANTS Cluster-crossover randomized clinical trial involving patients, family members, and clinicians from 36 adult ICUs with restricted visiting hours (<4.5 hours per day) in Brazil. Participants were recruited from April 2017 to June 2018, with follow-up until July 2018. INTERVENTIONS Flexible visitation (up to 12 hours per day) supported by family education (n = 837 patients, 652 family members, and 435 clinicians) or usual restricted visitation (median, 1.5 hours per day; n = 848 patients, 643 family members, and 391 clinicians). Nineteen ICUs started with flexible visitation, and 17 started with restricted visitation. MAIN OUTCOMES AND MEASURES Primary outcome was incidence of delirium during ICU stay, assessed using the CAM-ICU. Secondary outcomes included ICU-acquired infections for patients; symptoms of anxiety and depression assessed using the HADS (range, 0 [best] to 21 [worst]) for family members; and burnout for ICU staff (Maslach Burnout Inventory). RESULTS Among 1685 patients, 1295 family members, and 826 clinicians enrolled, 1685 patients (100%) (mean age, 58.5 years; 47.2% women), 1060 family members (81.8%) (mean age, 45.2 years; 70.3% women), and 737 clinicians (89.2%) (mean age, 35.5 years; 72.9% women) completed the trial. The mean daily duration of visits was significantly higher with flexible visitation (4.8 vs 1.4 hours; adjusted difference, 3.4 hours [95% CI, 2.8 to 3.9]; P < .001). The incidence of delirium during ICU stay was not significantly different between flexible and restricted visitation (18.9% vs 20.1%; adjusted difference, −1.7% [95% CI, −6.1% to 2.7%]; P = .44). Among 9 prespecified secondary outcomes, 6 did not differ significantly between flexible and restricted visitation, including ICU-acquired infections (3.7% vs 4.5%; adjusted difference, −0.8% [95% CI, −2.1% to 1.0%]; P = .38) and staff burnout (22.0% vs 24.8%; adjusted difference, −3.8% [95% CI, −4.8% to 12.5%]; P = .36). For family members, median anxiety (6.0 vs 7.0; adjusted difference, −1.6 [95% CI, −2.3 to −0.9]; P < .001) and depression scores (4.0 vs 5.0; adjusted difference, −1.2 [95% CI, −2.0 to −0.4]; P = .003) were significantly better with flexible visitation. CONCLUSIONS AND RELEVANCE Among patients in the ICU, a flexible family visitation policy, vs standard restricted visiting hours, did not significantly reduce the incidence of delirium.