2K took over publishing the NBA 2K series starting with NBA 2K6.
The first six games were published under Sega, beginning with the original NBA 2K on the Dreamcast in 1999. After Sega sold Visual Concepts to Take-Two Interactive, the new ownership structure led to the formation of 2K Sports and a publishing change.
NBA 2K6, released in 2005, was the first entry published by 2K rather than Sega. It was developed by Visual Concepts and appeared on PlayStation 2, Xbox, and Xbox 360. The game also marked the series’ first release on Microsoft’s Xbox 360.
Readers sometimes answer “2K Sports,” which is understandable because that branding was widely used for the series. The stored answer, “2K,” is the publisher name used for NBA 2K6 and is therefore the canonical answer to this question. The series has continued under 2K publishing.