The main objective of this study was to evaluate the reliability and the validity of a newly constructed reactive-agility test that can be used to define the reactive-agility performance in sports that employ repeated scenarios of "stop'n'go" agility. Measuring is done by original hardware based on the ATMEL microcontroller AT89C51RE2. A total of 36 college-aged male athletes (age, 22.1 ± 2.4 years; body height, 182.45 ± 5.19 cm; body mass, 80.67 ± 7.69 kg) and 21 college-aged female athletes (age, 21.4 ± 2.5 years; body height, 171.45 ± 6.81 cm; body mass, 61.95 ± 6.70 kg) were evaluated for a stop'n'go reactive-agility test (SNG-RAT), stop'n'go change of direction speed (SNG-CODS), countermovement jump, and anthropometrics. The reliability analyses suggested a high consistency for the applied tests (SNG-CODS, intraclass correlation coefficient [ICC] = 0.87 and 0.92; SNG-RAT, ICC = 0.81 and 0.86, for men and women, respectively). The SNG-RAT and SNG-CODS shared <40% of the common variance. The longer version of the SNG-RAT, which included 5 unpredictable changes of direction, was found to be valid among men. At the same time, the shorter version of the SNG-RAT (3 unpredictable changes of direction) is suggested for women because it better discriminates more agile from less agile athletes. Because the SNG-RAT and SNG-CODS were performed on the same course, we believe that simultaneously performing both tests can be beneficial when defining "stop'n'go" agility. This implies that the calculated SNG-CODS to SNG-RAT ratio will allow strength and conditioning coaches to indirectly determine the perceptual and reaction capacities of their athletes.