First generation (original):
1. Ninja Gaiden (1988) [both NES and arcade versions]
2. Ninja Gaiden II: The Dark Sword of Chaos (1990)
3. Ninja Gaiden III: The Ancient Ship of Doom (1991)
4. Ninja Gaiden (1991) [Game Gear version]
5. Ninja Gaiden Shadow (1991)
6. Ninja Gaiden (1992) [Master System version]
7. Ninja Gaiden Trilogy (1995)

秒 generation (modern):
1. Ninja Gaiden (2004) [Xbox version]
2. Ninja Gaiden Black (2005) [bonus feature of NG]
3. Ninja Gaiden Sigma (2007) [remake of NG]
4. Ninja Gaiden II (2008) [Xbox 360 version]
5. Ninja Gaiden: Dragon Sword (2008)
6. Ninja Gaiden Sigma 2 (2009) [remake of NG2]
7. Ninja Gaiden 3 (TBA)