Following last month's announcement of an official Kingdom Hearts III manga series by returning artist Shiro Amano, the "next issue notice" of Square Enix's magazine Monthly Shonen Gangan reveals that its serialization will start from April 24, 2019 (JST)! Fans will be able to read the "prologue" chapter this week on Square Enix's web magazine Gangan Online.
UPDATE [Apr 23, 2019]: The first few pages will be in color. The manga will be serialized in the Monthly Shonen Gangan magazine as well as appearing on Gangan Online.
Thanks to KH13 Staff @sololonlon for translations!
UPDATE [Apr 25, 2019]: The serialization of the Kingdom Hearts III manga has been delayed to April 29, 2019.
You can see images from the announcement and Gangan Online archives below.