That makes Ballmer one of Twitter's largest shareholders.
A Twitter account that identified itself as Ballmer's said Friday morning that he bought stock in Twitter in the last few months. Seth Burton, a representative for the Clippers, confirmed that Ballmer had made the investment.
The high-profile investment comes as Twitter is trying to win more users and turn a profit. This month, its co-founder Jack Dorsey returned as its permanent CEO. Then the company announced that it would lay off up to 8 percent of its workforce and unveiled a new feature, "Moments." Those packages of commentary, video and photos about major events are an attempt to make Twitter more accessible and broaden its appeal.
The company's shares lost about half their value between late April and late August as investors worried about Twitter's disappointing financial performance and slowing user growth. The nine-year-old company has never reported an annual profit.
The stock has recovered some of those losses as the broader markets rose in October, and is up $1.51, or 5.1 percent, to $31.22 in afternoon trading.
Ballmer also said he liked that Saudi billionaire Prince Alwaleed bin Talal and his investment company bought more shares of Twitter. The prince and his firm said this month the stake had doubled over a six-week period to more than 5 percent.
Dorsey was the San Francisco company's CEO during its early years and became interim CEO in July, when Dick Costolo stepped down. Twitter said on Oct. 5 that he would become its permanent CEO. Ballmer tweeted that day that Twitter "is remarkable" and called Dorsey an "impressive dude."
According to FactSet, Ballmer's holdings would make him the third-largest individual owner of Twitter shares after company co-founder Evan Williams and bin Talal. Ballmer, who was Microsoft's CEO for 14 years ending last year, is also the largest individual owner of Microsoft stock with a 4.2-percent stake.