Goniometry is commonly used to evaluate joint range of motion (ROM). The most widespread method, a manual universal goniometer (UG), is considered time-consuming and difficult to handle. The digital goniometer EasyAngle (EA) was developed to improve and simplify the evaluation of ROM. This study aimed to evaluate the reliability and validity of EA measuring active and passive flexion and extension of the knee joint. Reliability was assessed in a test-retest model, while validity was tested by comparing measurements from EA and UG. Thirty-six knee angles (18 individuals) were measured in standardized order. EA showed almost perfect reliability measuring active/passive flexion (ICC 0.994/0.994) and extension (ICC 0.978/0.987). EA's validity was very high in active/passive flexion (r = 0.969/r = 0.97) and moderate-to-high measuring active/passive extension (r = 0.751/r = 0.892). EA is a reliable, valid, and effective assessment tool for measuring knee angles. Further studies should verify these findings for other joints and movement directions.