Technical Abstract: The present experiments were conducted to field test a system optimized for on-line prediction of beef LM tenderness based on visible and near-infrared (VISNIR) spectroscopy and to develop and validate a model for prediction of tenderness that would be unbiased by normal variation in bloom time before application of VISNIR. For both Exp. 1 and 2, slice shear force (SSF) was measured on fresh steaks at 14 d postmortem. Carcasses with VISNIR-predicted SSF < = 15 kg were classified as VISNIR predicted tender and carcasses with VISNIR-predicted SSF > 15 kg were classified as VISNIR not predicted tender. In Exp. 1, spectroscopy was conducted on-line, during carcass grading, at three large-scale commercial fed-beef processing facilities with each carcass (n = 1,155) evaluated immediately after ribbing and again when the carcass was graded. For model development and validation, carcasses were blocked by plant and observed SSF and one-half of the carcasses (n = 579) were assigned to a calibration data set, which was used to develop regression equations, and one-half of the carcasses (n = 576) were assigned to a prediction data set, which was used to validate the regression equations. VISNIR predicted tender carcasses had lower (P < 10^-19) LM SSF values at 14 d postmortem in the calibration (13.9 vs. 16.5 kg) and prediction data sets (13.8 vs. 16.4 kg) than did VISNIR not predicted tender carcasses. Relative to the percentage of VISNIR not predicted tender carcasses, a lower percentage of VISNIR predicted tender carcasses had LM SSF > 25 kg in the calibration (2.0 vs. 7.8%) and prediction data sets (0.8 vs. 8.0%). In Exp. 2, carcasses (n = 4,204) were evaluated with VISNIR on-line at six commercial fed-beef processing facilities on 38 production d. VISNIR predicted tender carcasses had lower LM SSF values at 14 d postmortem (16.3 vs. 19.9 kg; P < 10^-87), longer sarcomere lengths (1.77 vs. 1.72 µm; P < 10^-10) and a higher percentage of desmin degraded (42 vs. 34%; P < 10^-5) by 14 d postmortem. Relative to the percentage of VISNIR not predicted tender carcasses, a lower percentage of VISNIR predicted tender carcasses had LM SSF > 25 kg (4.9 vs. 21.3%). The present experiments resulted in development and independent validation of a robust method to noninvasively predict LM tenderness of grain-fed beef carcasses. This technology could facilitate tenderness-based beef merchandising systems.