Capital Group earlier this month bought a big stake in rival Deutsche Bank, marking a vote of confidence in that troubled lender.
Shares in Commerzbank briefly jumped on the news of the Capital investment, making up some of its earlier losses, but were still trading 5% lower early afternoon in Frankfurt amid a global selloff.
Commerzbank declined to comment. Capital Group did not immediately respond to a request for comment.
The activist investor Cerberus also owns a big stake in both banks.
Both Commerzbank and Deutsche Bank were in talks to merge last year but called off the negotiations. Cerberus had been pushing for a merger, people with knowledge of the matter said at the time.
(Reporting by Tom Sims; Editing by Michelle Martin; editing by David Evans)