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Background Peripheral nerve blockade is a common analgesic technique in anterior cruciate ligament (ACL) reconstruction. The comparative opioid-sparing and functional outcomes of continuous femoral nerve block (CFNB) versus continuous adductor canal block (CACB) are unclear.Methods In this single-center, double-blind, prospective, superiority randomized controlled trial, 60 adults undergoing ACL reconstruction were randomized to receive either CFNB or CACB. All patients received a 20 mL loading dose of 0.5% ropivacaine and were discharged with a 0.2% ropivacaine infusion (5 mL/hour basal, 5 mL demand bolus, 30 min lockout). The primary outcome was average numeric rating scale (NRS) pain scores analyzed using a linear mixed-effects repeated-measures model over 48 hours. Secondary outcomes included cumulative morphine milligram equivalents (MME), continuous passive motion (CPM) usage, CPM compliance, and Quality of Recovery-15 (QoR-15) scores.Results 57 patients completed the study (CFNB n=28; CACB n=29). Mixed-effects repeated-measures analysis of average NRS pain scores at 24 and 48 hours showed no significant difference between CFNB and CACB. The estimated overall treatment effect for CFNB versus CACB was −0.31 NRS points (p=0.58, 95% CI −1.39 to 0.77), with no significant treatment-by-time interaction (p=0.97). Current NRS pain scores were lower for CFNB on postoperative day 0 (POD0) (p =0.02, 95% CI 6.44×10–6 to 2.00). In an exploratory analysis, fewer CFNB patients required more than 50 MME during the 48-hour follow-up period compared with CACB (CFNB: 5/28 vs CACB: 13/29; p=0.045, 95% CI 0.08 to 0.93). The absolute risk difference was 0.27 (95% CI −0.08 to 0.54). There were no differences in CPM use or compliance, bolus volume, or QoR-15.Conclusions CFNB was not superior to CACB on the primary pain outcome over 48 hours. In secondary analysis, CFNB catheters were associated with lower immediate postoperative pain. In exploratory analyses, fewer patients required more than 50 MME over 48 hours. This finding is hypothesis-generating and requires confirmation in an adequately powered trial. There were no differences in quality of recovery or CPM compliance. Whether any opioid-related differences between techniques outweigh concerns about motor blockade warrants further investigation.Trial registration number NCT03208478.