Mastery-oriented motivational climates and achievement goal orientations have been associated with a range of salutary and clinically relevant outcomes in both educational and sport research. In view of this, an intervention was developed for youth sport coaches designed to promote a mastery motivational climate, and a field experiment was conducted to assess its effects on changes in athletes’ achievement goal orientations over the course of a sport season. The experimental group was comprised of 155 boys and girls, who played for 20 basketball coaches; 70 youngsters played for 17 control group coaches. The coach intervention resulted in higher Mastery-climate scores and lower Ego-climate scores compared with the control condition, and athletes who played for the trained coaches exhibited significant increases in Mastery goal orientation scores and significant decreases in Ego-orientation scores across the season, whereas control group participants did not. Practical and theoretical implications of the findings are discussed.