This study was designed to examine the reliability and validity of the newly designed grip tool of the Baltimore Therapeutic Equipment (BTE)-Primus and to investigate the effects of body position (sit versus stand), handedness, and fatigue on grip strength. The subjects performed maximal grip strength tests using the Jamar dynamometer and the BTE-Primus. Intraclass correlation coefficients were calculated for test-retest reliability and criterion-related validity. A repeated measures analysis of covariance was conducted to reveal differences in grip strength between instruments, body positions, hands, and sessions. The BTE-Primus grip tool was found to be reliable (r = 0.97 to 0.98) and valid (r = 0.95 to 0.96). There were no significant differences in grip strength scores between the Jamar and the BTE-Primus or between sitting and standing. Grip strength scores of the right hand were significantly greater than those of the left hand, and grip strength scores in the first session were significantly greater than those in the second session. The results of this study indicate that clinicians can use the BTE-Primus grip attachment at the second handle setting and know that is reliable, valid, and comparable to the second-handle setting of the Jamar dynamometer.