© 2018 Guo et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Background Cardiorespiratory fitness (CRF) is the only major risk factor that is not routinely assessed in the clinical setting, for preventive medicine. A valid and practical CRF test is needed for use in the clinics. The objective of this study is to demonstrate the validity of a 3-minute squat test to assess CRF in primary care. Methods A cross-sectional study in which the participants performed both the Ruffier squat test and the Balke maximal treadmill test. The study was conducted in a clinical setting from September 2016 to March 2017. We recruited a convenient sample of 40 adults between 18 and 64 years from the general U.S. population. Participants completed 30 squats in 45 seconds, paced by a metronome. Heart rate was measured at rest (P1), immediately after the test (P2), one minute after the test (P3). VO _2max was measured using the Balke maximal treadmill fitness test. Results Of the 40 participants, there were 18 men and 22 women. Mean age was 31.2 years (SD = 9.9). We found that the best VO _2max predictors were HR features P1/height and (P2–P3)/ age3. Our best-performing model using these two features predicted individuals’ CRF levels with an adjusted R2 of 0.637, sensitivity of 0.79, and specificity of 0.56. Conclusions The study provided strong evidence for the validity of the squat test in the clinical setting. Further, the equation of our model along with VO _2max normative tables provides an efficient and easy way to assess CRF in a primary care setting.