In a nutshell, the blood-coloured spots are a side-effect of cupping, a healing technique which is said to increase blood flow and help sore muscles to recover.
Cupping essentially involves a series of cups and pumps, which are attached to the body before suction is created, as shown by Belarusian swimmer Pavel Sankovich.
Although it is seen as an ancient form of therapy, it appears Team USA have long adopted this treatment, with swimmer and 12-time Olympic medallist Natalie Coughlin sporting the marks over two years ago.
Current US gymnast Alex Naddour displayed the mark on a recent Instagram post, proving it's not just swimmers who are using this recovery aid.
The bruises can last for up to two weeks according to The State News, who investigated Michigan State University's use of cupping.
Whether it catches on remains to be seen, but expect these spots to be dotted around numerous athletes before the closing ceremony.