Brittney Griner has asked her supporters in a social media post to advocate for Paul Whelan.
Whelan is a former Marine serving a 16-year prison term in Russia for espionage charges the United States are baseless.
Griner is a WNBA star who was detained in Russia in February and released in December in a prisoner swap for Russian arms dealer Viktor Bout.
The U.S. had sought to exchange Bout for both Whelan and Griner, but Russia would only agree to swap Griner.
In a handwritten letter posted to Instagram, Griner urged her supporters to advocate for Whelan and send letters to him.