Microsoft and Rare announced that the Closed Beta for Sea of Thieves will begin on January 24.
The Sea of Thieves Closed Beta will begin at 4am PT on January 24, and end at 12:00am PT on January 29. This Closed Beta will not be the full game, which was done to keep a good selection of things under wraps for players to discover at launch.
This Closed Beta will be accessible to anyone who joined the Sea of Thieves Insider Program before December 1, 2017, and will also be available to anyone who pre-orders the game.
Note that the Closed Beta will not be under NDA.
Source: Press Release