Megasoft (株式会社メガソフト) was a wholly-owned subsidiary of Sega. Originally established as Whiteboard (ホワイトボード), it was later renamed Santos (サントス). Santos was acquired by Sega in December 1991 and changed names again. Megasoft was merged into Sega in November 1992  during the production of the two games listed below.
Takeshi Tozu (戸津猛) was the president of Whiteboard/Santos. (He previously headed Sesame Japan and Crux. ) He was replaced after the Sega acquisition (apparently by Tokinori Kaneyasu [兼安時紀] from Data East ) and apparently started another company called Santos with some previous employees and intellectual property.
Gai Brain was started by former staff.
- Airwolf (Kyugo) 
- Dakko-chan House (Sega)
- Photo Mahjong: Gekisha (Whiteboard)
- Sukeban Janshi Ryuuko (Sega)
- Airwolf (sound?) (JP Publisher: Kyugo)
- Programming: C-lab.?
- Space Harrier (JP Publisher: Takara)
- Started out as a game based on Licca-chan, Takara's popular fashion doll
Mark III/Master System
- Alex Kidd: The Lost Stars (JP/US/EU/AU Publisher: Sega; BR Publisher: Tec Toy)
- Danan: The Jungle Fighter (EU/AU Publisher: Sega; BR Publisher: Tec Toy)
- Dynamite Dux (EU/AU Publisher: Sega; BR Publisher: Tec Toy)
- Masters Golf / Great Golf (JP/US/EU/AU Publisher: Sega)
- Megumi Rescue (JP Publisher: Sega)
- Nekkyuu Koushien (JP Publisher: Sega)
- Opa Opa / Fantasy Zone: The Maze (JP/US/EU/AU Publisher: Sega; BR Publisher: Tec Toy)
- Reggie Jackson Baseball / American Baseball (US/CA/EU/AU Publisher: Sega; BR Publisher: Tec Toy)
Artist Ano Shimizu worked on Dynamite Dux and Nekkyuu Koushien at Whiteboard.  A code comparison between the two revealed shared code in the rest of the games on this list except Alex Kidd and Great Golf. Alex Kidd, Fantasy Zone, Great Golf, Megumi Rescue, and Nekkyuu Koushien contain the same initials, "YM," in the header.  Additionally, the "standing" screen in Great Golf contains the name "KEI M," likely a reference to Whiteboard programmer Kei Maruyama. 
- Mahjong Cop Ryuu: Hakurou no Yabou (JP Publisher: Sega)
- Hammer Away (unreleased) (Sega)
- Went on location test but was cancelled
- Mahjong Quest (Taito)
- Space Harrier (JP/US/EU Publisher: Sega; BR Publisher: Tec Toy; KR Publisher: Samsung)
- Turns up on a list of Sega games, but there's no other evidence at this time
- Battle Golfer Yui (JP Publisher: Sega)
- Toki: Going Ape Spit / JuJu Densetsu (US/EU/JP Publisher: Sega; BR Publisher: Tec Toy; KR Publisher: Samsung)
- Janshin Densetsu [MVS] (Yubis)
- This was developed around 1991 and dusted off for a 1994 release.
- Aa Harimanada (JP Publisher: Sega)
- The Super Shinobi II / Shinobi III: Return of the Ninja Master (JP/US/EU Publisher: Sega; BR Publisher: Tec Toy; SK Publisher: Samsung)