The Developmental Player System (育成選手制度, Ikusei Sensyu Seido) is a system that allows players to be held for the purpose of training players separately from the 70 registered players under control of each team created in the fall of 2005 at Nippon Professional Baseball (NPB).
 This system was established in 2005 to create an environment in which amateur players can newly play in the situation where company-owned amateur baseball teams were being abolished one after another. A Developmental player (育成選手, Ikusei Sensyu) is a player who aims to improve baseball skill training and manner training in order to aim for registered players under control, that is eligible to participate in NPB's first league official game (Central League, Pacific League, Interleague play (NPB)). (Regulations for NPB Developmental players, Article 2) 
 A developmental players include players drafted as rookies (e.g. Yusuke Kosai and Michitaka Nishiyama, drafted developmental player who were first signed as a registered player under control in 2006.), foreign players signed as developmental players (e.g., Raidel Martínez, Liván Moinelo.), players drafted as registered players under control but re-signed with a team as developmental players due to their abilities (e.g., Kenta Kurose - Re-signed as a registered player under control.), players who became free agents with their former team and signed with another team (e.g., Masaru Nakamura, Kouya Fujii - Re-signed as a registered player under control.), and players who were removed from the registration of players under control and re-signed as a developmental player due to treat injuries (e.g., Takuya Kuwahara, Hiroya Shimamoto.). There is an argument that eliminating players who cannot play due to injury from the registration of players under control and re-signing them as developmental players will protect their careers. On the other hand, the use of the developmental players system instead of the Major League Baseball (MLB) Injured list system is criticized as contrary to the original principle of this system, which is to open the door to amateur players and develop them. The oldest player to become a developmental player is Soichi Fujita, who signed in 2011 at the age of 39. The longest period as a developmental player was Kousuke Naruse [jp]'s eight seasons (2011-2018) as of the 2022 season.
 The developmental player system, which began in the 2005 season, has been successful in producing major players for each team. (e.g. Yoshiki Sunada, Yuji Nishino) The Yomiuri Giants, who have been working on this system since early on, have signed more than 50 developmental players as their registered players under control as of the 2022 season. The Fukuoka Softbank Hawks have also signed 40 developmental players as their registered players under control, and have produced players like Kodai Senga and Takuya Kai who have won league titles and awards and represented Japanese baseball on the national baseball team. The Hokkaido Nippon-Ham Fighters, which had been reluctant to acquire developmental players, drafted a player as a developmental player for the first time in the 2018 draft. In the 2022 draft, 51 players will be selected as developmental players, the largest number in history, and the acquisition of developmental players is a growing trend.