Google parent Alphabet Inc. said Eric Schmidt will step down from his post as executive chairman in January, an unexpected retreat by the tech giant’s most high-profile ambassador.

Mr. Schmidt, who joined Google in 2001 as chief executive and served in that position until 2011, will transition to a role as technical adviser and will continue to serve on Alphabet’s board, the company said in a statement Thursday. Alphabet said his replacement will likely be a nonexecutive chairman.

Google said little about the reason for the change, and declined to make Mr. Schmidt, 62 years old, available for an interview. In the statement, he said that co-founders Larry Page and Sergey Brin, Google CEO Sundar Pichai “and I all believe that the time is right in Alphabet’s evolution for this transition.”

The Google co-founders and Mr. Schmidt have been discussing his transition for about a year, according to a person close to the company.

Mr. Schmidt’s retreat signals a generational shift at the tech giant, which he helped shape into one of the world’s most valuable and influential companies. Messrs. Page and Brin had brought Mr. Schmidt on board when Google was still a scrappy search engine, and the older executive gave Google structure and discipline.