Based on the importance of fundamental movement skills (FMS) and common performance traits of persons with developmental disabilities (DD) that impact the performance of FMS, a rater training is necessary to evaluate FMS accurately among this population. The first purpose of this study was to develop a rater training protocol on the Test of Gross Motor Development-3 (TGMD-3) for novice raters. The second purpose was to validate this rater training protocol using a modified Delphi method. The third purpose was to examine novice raters’ accuracy in scoring the motor skills of children with DD after completing an online rater training protocol. A total of eight experts completed two rounds of a modified Delphi method with intraclass coefficient statistic (ICC) of .75 or above providing evidence of consensus on content and presentation of training material. A total of 41 novice raters completed three rounds of training and scoring separated by 5 days each. Data analysis compared the change of rating accuracy of novice raters with that of experts on the run and two-hand strike skills on the TGMD-3 across three different occasions. There was a significant impact on scoring accuracy of novice raters to score the run (F(1, 39)= 56.431, p < .001), two-hand strike skills (F(1, 39)= 35.549, p < .001), and the total skill score (F(1, 39)= 64.323, p < .001). The TGMD-3 online rater training program for novices in the present study provides a model training program to improve the accuracy of scoring FMS among children with DD.