Manga

That Time I Got Reincarnated as a Slime (転生したらスライムだった件) is a manga adaptation based on the light novels of the same name. In addition to the main story, there are also side-stories and spin-offs based on the series.

Main
The main manga is illustrated by Taiki Kawakami and debuted in Kodansha's Monthly Shōnen Sirius on March 26, 2015. The series is compiled into several tankōbon volumes, with the first volume released on October 30, 2015. Kodansha USA announced their license to the manga during their panel at New York Comic Con on October 6, 2016. They released the first volume in English on August 22, 2017.

Chapters Not Yet In a Volume

 * 53: Myulan the Witch
 * 54: A Prelude to Catastrophe
 * 55: The Natural Enemy of Monsters
 * 56: Intertwining Thoughts
 * 57: Malicious Visitors

How the Monster Country Works
That Time I Got Reincarnated as a Slime: The Ways of Strolling in the Demon Country (転生したらスライムだった件～魔物の国の歩き方～) is a side-story manga by Shō Okagiri. The series has been serialized on Micro Magazine's Comic Ride website since July 28, 2016. The first volume was released on April 7, 2017.

Reincarnated Slime Diary
Slime Diary: That Time I Got Reincarnated as a Slime (転スラ日記 転生したらスライムだった件), often shortened to Reincarnated Slime Diary (転スラ日記), is a 4-koma spin-off manga based on the main series. It is illustrated by Shiba and began serializing on Shonen Sirius. The first volume released on September 28, 2018.

That Time I Got Reincarnated as a Corporate Slave
That Time I Got Reincarnated as a Corporate Slave (転生しても社畜だった件) is a spin-off manga illustrated by Shizuku Akechi. It has been serialized on the Suiyōbi no Sirius web magazine since September 26, 2018, and it is also been serialized in print in Kodansha's Monthly Shōnen Sirius since October 26, 2018. The story centers on Rimuru, who re-awakens to the real world in a salaryman job similar to his previous life as Mikami, but still in his slime body.

Trinity of the Magic Kingdom
That Time I Got Reincarnated as a Slime Another Story: Trinity of the Magic Kingdom (転生したらスライムだった件 異聞 ～魔国暮らしのトリニティ～) is a spin-off manga illustrated by Tae Tono. It was first published on Monthly Shonen Sirus magazine's "Suiyōbi no Sirius". The manga centers on Phos, a girl from the Beast Kingdom of Eurazania. Phos goes to Tempest on secret orders from Demon Lord Carrion.

Trivia

 * Out of the various manga adaptations, four are currently released on Monthly Shōnen Sirius.