The purpose of the present study was to develop a Projection Index for the Rorschach test that reflects the degree to which an examinee’s responses are influenced more by their individual characteristics than by the properties of the cards. Using Rorschach data from 922 adults, we comprehensively utilized key scoring variables from the Rorschach Comprehensive System (CS) as well as major verbal response information from both the response phase and the inquiry phase. To construct the Projection Index based on relevant variables, correlations were computed for all 11 scoring variables. The maximal combination of variables showing correlations r ≥ .20 was selected, resulting in five final variables: “verbal responses in the response phase,” “verbal responses in the inquiry phase,” “determinants,” “content,” and “special scores.” Exploratory factor analysis yielded a single factor, which explained approximately 34% of the variance. This one-factor structure was also confirmed through confirmatory factor analysis, with the model showing good fit indices. The Projection Index showed positive correlations with the S-CON, DEPI, PTI, HVI, and OBS indices. Two-way ANOVA results indicated a significant interaction effect of the CDI and adjD variables on the Projection Index. When the adjD score was below zero, the Projection Index did not differ markedly regardless of whether the CDI score was ≥ 4. However, when the adjD score was zero or higher, the Projection Index differed substantially depending on whether the CDI score was ≥ 4. When integrated with the Rorschach CS variables, the Projection Index developed in the present study is expected to make a significant contribution to advancing evidence-based projective interpretation techniques.