Cryptocurrency investor and entrepreneur Justin Sun apologized on Chinese social media for what he described as “excessive self promotion,” after a tumultuous few days following his postponement of a charity lunch with Warren Buffett this week.

“I feel deeply ashamed of my excessive self-promotion as well as my penchant for hyping up things,” the 29-year-old said in a long post in Chinese on Weibo, a Twitter-like microblogging site in China.

Mr....