The Norfolk, Va.-based group purchased shares in SeaWorld to push changes at the park, including the release of some of the animals for which the theme park is famous.
A column on PETA's blog explains their reasoning.
"We quickly purchased the smallest number of shares necessary to give us the right to attend and speak at annual meetings, and to submit shareholder resolutions asking for policy changes," Michelle Kretzer, a PETA blogger, wrote.
"Our first order of business as part-owners of SeaWorld? Getting the orcas out -- including Corky, who has been enslaved by SeaWorld for 44 years."
The blog did not disclose the amount of shares PETA purchased, except that it was enough to be present in shareholder meetings.
SeaWorld priced the company's initial public offering of 26 million shares of common stock at $27 per share, according to the San Antonio Business Journal. Shares are trading under the ticker symbol SEAS.