For athletes and training populations, the ability to track progress and determine appropriate workloads to stimulate adaptation is vital. Measures that represent a person’s aerobic fitness such as VO2 max and blood lactate concentration are often used for these purposes. While these measures can reveal a lot about a person’s aerobic fitness, they also require invasive methods and/or testing procedures that require a lot of time and money. Critical speed (CS) is an indicator of aerobic fitness that represents the greatest metabolic rate that results in the rate of lactate production being matched by the rate of clearance. It also precisely predicts aerobic performance and tolerance. Testing for critical speed traditionally required multiple bouts to exhaustion, until the 3-minute testing model was developed. Though the validity of a 3-minute running test using minimal equipment has been shown, the purpose of this study was to assess the between-day and inter-rater reliability of this test. Sixteen healthy participants completed an all-out 3-minute running test and served as a rater for one other participant. Eleven participants also completed a second all-out running test 3-14 days after the initial session. From time splits recorded by the participants (Session 1) and an expert rater (Sessions 1 and 2), CS was calculated and inter-rater and between-day reliability assessed. The primary findings of this study are that the use of the 3-minute running test as applied in this study requires no specialized training as evidenced by the fact that CS values obtained by individuals with minimal training were consistent to those obtained by an expert rater. However, while the test-retest reliability of the 3-minute test was strong-excellent (ICC = 0.82), there was a systematic bias detected in which participants exhibited significantly greater CS values during Session 2 compared to Session 1, and the magnitude of this difference (0.41 m/s) was determined to be clinically meaningful. Therefore, it was concluded that though the test could be conducted consistently by coaches, physical education teachers, or personal trainers with minimal familiarization and equipment, the use of this test for determining critical speed without the application of greater testing controls is not supported due to the fact that the magnitude of the difference in CS between testing sessions was large enough that it would lead to different interpretations of an individual’s cardiovascular fitness.