This study investigates the relationship between mastery goal performance and achievement on the State of Texas Assessments of Academic Readiness (STAAR) test. With policies like the Every Student Succeeds Act (ESSA) placing increased importance on standardized-state assessments, schools are making efforts to improve scores, often by focusing heavily on test preparation. However, such conventional methods can restrict teaching in unproductive ways. Mastery goals, based on achievement goal theory, offer a potential alternative, although limited research exists on their impact on testing outcomes. The researcher evaluated a mastery goal intervention for 224 students in grades 3-8 from a Texas school using a quantitative paired samples t-test. Students were given personalized mastery goals based on the practice STAAR test administered two months before the 2019 STAAR tests. Results showed significantly higher 2019 STAAR test scores compared to the practice STAAR test, with math scores increasing an average of 11.3 points and reading scores by 4.9 points. Positive effects occurred across ethnic subgroups, grade levels, genders, and special services (special education, limited English proficiency, Section 504 and gifted and talented). This research adds to the growing number of studies on achievement goal theory by quantifying the links between personalized mastery goals and high stakes testing outcomes. Personalized mastery goals show promise for practical application in schools to improve performance on tests like the STAAR. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]