The main concern of the present study was to investigate the probable correlation between the bilinguals’ second language grammatical proficiency level and their third language grammatical proficiency level. The current study was implemented on selecting a total of 100 Iranian female high school students studying at second grade from two educational districts of Tabriz (an Azeri-speaking city in Iran). The participants were homogeneous in terms of their English proficiency level (achieved through the Oxford Solutions Placement Test), sex, and age. They attended public high schools; they were taught the same material and they had the same number of hours of instruction. Two grammatical proficiency tests (Persian and English), a background questionnaire were administered to the participants. Statistical analyses including descriptive statistics and a Spearman’s Rho Correlation revealed a significant correlation between the bilinguals’ second language grammatical proficiency level and their third language grammatical proficiency level. Finally, the implications of the results as they relate to educational programs were discussed.