Background: While numerous articles on Criterion® have been published and its validity evidence has accumulated, test users need to obtain relevant validity evidence for their local context and develop their own validity argument. This paper aims to provide validity evidence for the interpretation and use of Criterion® for assessing second language (L2) writing proficiency at a university in Japan. Method: We focused on three perspectives: (a) differences in the difficulty of prompts in terms of Criterion® holistic scores, (b) relationships between Criterion® holistic scores and indicators of L2 proficiency, and (c) changes in Criterion® holistic and writing quality scores at three time points over 28 weeks. We used Rasch analysis (to examine (a)), Pearson product--moment correlations (to examine (b)), and multilevel modeling (to examine (c)). Results: First, we found statistically significant but minor differences in prompt difficulty. Second, Criterion® holistic scores were found to be relatively weakly but positively correlated with indicators of L2 proficiency. Third, Criterion® holistic and writing quality scores--particularly, essay length and syntactic complexity--significantly improved, and thus are sensitive measures of the longitudinal development of L2 writing. Conclusion: All the results can be used as backing (i.e., positive evidence) for validity when we interpret Criterion® holistic scores as reflecting L2 writing proficiency and use the scores to detect gains in L2 writing proficiency. All of these results help to accumulate validity evidence for an overall validity argument in our context.