The Japanese Olympic Committee mistakenly used the photo of China's synchronized swimming team instead of Japan's in its annual calendar for 2017, an informed source said Saturday.
It has come to light after someone with the Japanese national team pointed out to the JOC, and the Japan Swimming Federation said modified versions have been redistributed.
The JOC had asked the federation to check the photos in previous years, but that hadn't happened this time.
Japan won the bronze medal and China the silver in both duet and team at the Rio Olympics last summer.
"There's no fault on the JSF's part. It is troubling they mistake Olympic athletes, the medalists," JSF senior director Kaname Sakamoto said.
A JOC senior official said, "It's all our mistake in checking. We're sorry."