Background: Peripheral intravenous catheters are the most widely used invasive device in hospitals but have serious risks. Objective: To determine if a structured assessment and decision tool (I-DECIDED®) improves daily peripheral intravenous catheter assessment and care decisions. Design: Prospective, interrupted time-series study. Settings: Seven adult inpatient wards in three Australian hospitals. Participants: 825 adults with 867 peripheral intravenous catheters. Methods: Between August 2017 and December 2018, peripheral intravenous catheter assessments and chart audits were undertaken with informed patient consent. Following a 4-month pre-intervention period (with 2-weekly measures), the I-DECIDED® tool was implemented over 3 months (no data collection) using multiple strategies (stakeholder meetings, vascular access device form, education sessions, ward champions, lanyard cards, and posters), followed by a 4-month post-intervention period (with 2-weekly measures). Primary outcomes were device utilization (number of peripheral intravenous catheters per total number of patients screened); idle/unused catheters; insertion site complications, substandard dressing quality; and primary bloodstream infections. Results: Of 2055 patients screened, 1175 (57.2%) had a peripheral intravenous catheter, and 825 patients (867 catheters) consented and were included in the final analysis. Device utilization increased from 42.0% of catheters at baseline to 49.6% post-intervention (absolute risk difference [ARD] 7.5%, 95% confidence interval [CI] 4.8, 10.3; relative risk [RR] 1.18, 95% CI 1.11, 1.25; p < 0.001). The proportion of idle catheters reduced from 12.7% to 8.3% (ARD − 4.4%, 95% CI − 8.5, − 0.3; RR 0.66, 95% CI 0.44, 0.97; p = 0.035). Peripheral intravenous catheter complications reduced from 16.1% to 10.9% (ARD − 5.2%, 95% CI − 9.7, − 0.6; RR 0.68, 95% CI 0.48, 0.96; p = 0.026). Substandard dressings reduced from 24.6% to 19.5% (ARD − 5.2%, 95% CI − 10.7, 0.4; RR 0.79, 95% CI 0.61, 1.02; p = 0.067). Only one primary bloodstream infection occurred (post-intervention). Conclusions: Implementation of a comprehensive device assessment and decision tool (I-DECIDED®) reduced idle catheters and catheter complications, despite higher device utilization. Dressing quality improved but was not statistically significant. Further implementation of the tool could improve hospital safety for patients with an intravenous catheter. ANZCTR trial registration: ACTRN12617000067370. Date of registration 13 January 2017. Date of first data collection 3rd August 2017. Tweetable abstract: #IDECIDEDassessment reduces prevalence of idle peripheral catheters and device complications.