TUI Travel PLC, one of Europe's largest tourism companies, has acquired 8.4 million shares in Air Berlin and now owns 9.9% of the airline.
Air Berlin is Germany's second-biggest airline behind Lufthansa.
TUI Travel already owns Germany’s third-biggest airline, TUIfly.
In September, Air Berlin and TUIfly scrapped plans for a cross-shareholding deal. However, the two carriers did forge ahead with a deal in which Air Berlin assumed operations of TUIfly's city routes, enabling TUIfly to focus on the vacation market.