In this paper, I point out Thābit ibn Qurra’s usage of Indian astronomical and mathematical knowledge in his works, showing that he learned very well, not only Greek mathematics and astronomy, but also Indian astronomy and mathematics. Then, I elucidate, with recourse to his biographical information, why he knew Indian astronomy and mathematics. In al-Ma’mūn’s days, scholars in the court became interested in Greek science, and owing to their growing interest, those who were engaged in it competed with each other in their knowledge for positions at court. As a result, they tried to expand their research groups by recruiting people in order to increase their knowledge of Greek science. This expansion gave promising persons the opportunity to build connections with prominent scholars at court, and, for Thābit, the chance to learn Greek science at the Banū Mūsā’s home. In the course of working with the Banū Mūsā family, he rapidly amplified his knowledge of Greek science, using the abundant resources that they had collected, eventually becoming the top scholar in the field. Since Indian astronomy and mathematics were also necessary knowledge for scholars engaging in Greek science in Baghdad, Thābit learned them also with the family. Therefore, there is no surprise that he mastered not only Greek astronomy and mathematics but also their Indian counterparts.