Dorsey tweeted about the stake on Valentine's Day, appropriately adding the hashtag "Love Twitter."
Dorsey purchased about 426,000 of the shares, at prices from $15.84 to $16.60, adding up to around $7 million.
Separately, Dorsey has sold more than $11 million worth of shares in Square, the other company where he serves as CEO, since the beginning of 2017, as part of a pre-scheduled trading plan.
Twitter's shares are down more than 9.5 percent over the past week, after the company reported earnings guidance that fell far short of what Wall Street expected for the fourth quarter of 2016. But during the earnings call, Dorsey, who is also the CEO of Square, reiterated his support for the company.
"Late last year we really flattened the organization, so that we could elevate engineering product and design, so they report directly to me, so I could be a lot closer to the products," Dorsey said. on the call "Now that we spent a year really going through and making sure that we reset the foundation on what we're executing, what our priorities are and how we execute from an engineering perspective, we have a lot more confidence that we can move a lot faster on bigger things."
It's not unusual for big players within companies, like Dorsey, to buy or sell their shares on a scheduled basis, but Dorsey's recent purchase did not mention a trading plan.
CNBC reached out to Twitter for comment.