Twitter Inc. co-founder and board member Evan Williams disclosed Thursday that he plans to sell up to 30% of his shares in the social-media company over the next year.

Mr. Williams, Twitter’s largest individual shareholder with a 6% stake, cited personal reasons for the selling plan, which kicked off Monday. He said he plans to remain on the company’s board and then keep the “vast majority of my assets” tied up in Twitter shares.

“It actually pains me to be selling at this point, but this sale is all about personal context, not company context,” Mr. Williams wrote in a post on Medium, the blogging startup he now runs as chief executive.

After selling roughly $4 million worth of stock this week, Mr. Williams’s current stake is worth about $623 million at the current share price. This week’s sale accounted for less than 1% of his holdings in Twitter, according to securities filings.

“The plan is not expected to result in the sale of more than 30% of Ev’s holdings,” a Twitter spokeswoman said in a statement. She said the sales are part of Mr. Williams’ “continued efforts to diversify his financial positions and focus on his charitable organization and other endeavors.”