AlphaDream Corporation, Ltd.(JP)(CN) was an independent developer from Japan. For much of its lifespan, the company was dedicated to producing video games for the handheld Nintendo consoles.
The company was especially renowned for producing Mario & Luigi titles on behalf of Nintendo, a series of role-playing games centred around the two plumbers with elements of slapstick comedy. Starting with Mario & Luigi: Superstar Saga in 2003, the series became a critical success with the Mario fanbase.
Despite the success of the Mario & Luigi series, the company had filed for bankruptcy on October 1st of 2019 and shut down. 
The company was founded as Alpha Star in 2000 by Tetsuo Mizuno, a past president of Square, with several employees coming from Square. Their first game, Koto Battle, was released the following year for the Game Boy Color; it was in the vein of the Pokémon series, though more complex and a bit more card-based.
This was followed up with Tomato Adventure for the Game Boy Advance in 2002, a role-playing game based on the theme of tomatoes and Hamtaro: Rainbow Rescue a year later. So far during the company's infancy, it was relatively low-profile with only a couple of Japanese-exclusive titles. However, the company had their first brush with Nintendo during that period, as Chihiro Fujioka - a Square employee who was a director of Super Mario RPG - assisted with the development of Tomato Adventure together with Graphic Design.
Soon enough the company broadened their horizons substantially when Nintendo struck a deal with them to work on a Mario & Luigi role-playing game, which was released to the shelves in the fall of 2003 as Superstar Saga. The title was an immense success, with AlphaDream establishing themselves as a competent developer with a burgeoning reputation for being a very co-operative developer to work with. It was reported that Nate Bihldorff of Nintendo Treehouse once commented that AlphaDream was an excellent team to work with because of how fun they were. He explained that they knew the Mario series extremely well and thus were a perfect fit for creating the Mario & Luigi series.
Barring a couple more games in the Hamtaro range and Japanese one-shots, for the rest of their lifespan AlphaDream continued to develop three more Mario & Luigi titles as the series flourished, peaking with Bowser's Inside Story in 2009. After crossing together Mario & Luigi with Paper Mario to produce Paper Jam in 2016, the company delivered two remakes of their previous Superstar Saga and Bowser's Inside Story iterations for the 3DS console in 2018, which both included extra content. However, it was reported that the remakes were a commercial flop to the extent of being the worst-selling games in the Mario franchise, with the latter reportedly selling a feeble 20,000 copies in Japan two months after its launch. This was not helped by the 3DS system's popularity having long been on the decline, especially after the release of the next-gen Nintendo Switch.
On top of that, the company was saddled with a whopping debt of ¥465 million in March 2018, equivalent to $3.7 million.  This was reportedly due to sluggish revenues and increase of development costs that proved unsustainable in the long term, further worsened by the remakes selling poorly in the game market. The company had struggled to operate under these conditions and on the 1st of October 2019, it eventually filed for bankruptcy.