PURPOSE: The 6-min walk test is a simple clinical outcome measure, which has been used frequently to assess functional performance in many different patient groups, including patients with total knee arthroplasty (TKA). The 6-min walk test measures the maximal distance a subject is able to walk in 6 min. The reliability is unknown in patients with TKA. Therefore, the purpose of the study was to assess the reliability of the 6-min walk test in patients with recent TKA. METHODS: Thirty-four patients with TKA performed 2 test trials the same day, separated by a 1-h seated rest. To assess reliability, intra-class correlation coefficient (ICC(2,1)), standard error of measurement (SEM), and smallest real difference (SRD) were calculated. RESULTS: The patients walked on average 14.1 m longer at the second (397.2 m) compared to the first (383.1 m) test trial. The ICC(2,1), SEM, and SRD were 0.97, 13.0, and 36.1 m, respectively. CONCLUSIONS: The intra-tester reliability of the 6-min walk test was high in patients with TKA. The thresholds of the 6-min walk test to detect a real change are acceptable in research (SEM) and clinical settings (SRD). We recommend that the longest distance walked in 2 supervised test trials should be used. LEVEL OF EVIDENCE: I.