The agreement between a wearable inertial sensor (GYKO, G) and the force platform (P) was assessed by evaluating "test-retest" and "inter-rater reliability." Thirty-eight subjects were enrolled; the selected indices of balance were investigated over foot positions and (un)stable conditions. Intraclass correlation coefficient (ICC), standard error of measurement (SEM[subscript %]) and minimal detectable change (MDC[subscript95%]) were computed. For G, ICC bounds range from "poor" to "excellent" (0.040 ÷ 0.921), mean of 0.687 (= "moderate reliability"). Regarding P, ICC ranges from "poor" to "excellent" (0.070 ÷ 0.920), mean of 0.683 (= "moderate reliability"). For G, SEM[subscript %] ranges from 11% to 47%; MDC[subscript95%] from 30% to 132%. Concerning P, SEM[subscript %] ranges from 7% to 41%; MDC[subscript95%] from 21% to 114%. Finally, the "inter-rater reliability" ICC by comparing devices ranges from "poor" to "excellent" (-0.162 ÷ 0.911), mean of 0.338 (= "poor reliability"). GYKO appears to be a convenient tool with high consistency among multiple measurements but for specific clinic/research purposes.