Citing a Securities and Exchange Commission filing made public on Thursday, the Post said Gate's private investment group, Cascade Investment LLC, along with the Bill and Melinda Gates Foundation, purchased a total of 13.5 million shares of Cox Communications on Oct. 29.
The newspaper said sources familiar with the transaction said the stake was acquired as a passive investment and would not provide either Gates, his family members or representatives with any management role in Cox, the fifth largest U.S. cable company.
Tax law prevents a foundation from owning more than 20 percent of a corporation. The Gates Foundation's 5.7 million shares make up less than 1 percent of Cox's 600 million outstanding shares, according to the Post.
The newspaper reported the investment came as Gates was actively supporting the company's bid for AT&T Corp.'s cable unit, a battle that was eventually won by Comcast Corp.
Cox said at the time that it had sold stock to two private investors but did not reveal the names, the newspaper said.
It quoted a Cox spokesman as saying that the deal was not related to the AT&T bid.
The Post said Cascade did not return a call seeking comment. The Gates foundation said it does not discuss its investment strategy.
Cascade acquired 8.1 million shares of Cox Communications for $299.4 million and the foundation acquired another 5.4 million shares for $199.6 million, the Post said, citing information provided by Cox.
The newspaper said over the years, Gates and his company have become one of the cable industry's biggest investors.
In addition to his personal stake in Cox, Microsoft has a $5 billion investment in AT&T's cable division, the largest U.S. cable company.
In 1995, Gates invested $1 billion in Comcast -- a stake that has since quadrupled in value, the report said.