Brittney Griner Calls For Fans To Send Support Letters To Paul Whelan

Brittney Griner is asking everyone who supported her while in Russian custody to do the same for Paul Whelan ... by sending encouraging letters to the former U.S. Marine.

BG emphasized how much messages helped her throughout the past 10 months she was behind bars in Russia ... saying in an Instagram post it was one of the few gestures that kept her hope alive.

"Your letters were also bigger than uplifting me," the WNBA champ said. "They showed me the power of collective hands. Together, we can do hard things. I'm living proof of that."

"My family is whole and now, thanks to you, we are fortunate to get to spend the holidays together. However, there remain too many families with loved ones wrongfully detained."

The 32-year-old specifically brings up the former Marine, who's serving a 16-year prison sentence in Russia for an espionage charge in 2018, saying she wants people to show the same amount of love for Whelan as he remains behind bars.

"I hope you'll join me in writing to Paul Whelan and continuing to advocate for other Americans to be rescued and returned to their families," Griner said.

Of course, Whelan -- as well as many Americans -- is displeased with President Joe Biden and the administration for not including him in the prisoner swap for Griner earlier this month .... especially since he claims he "was arrested for a crime that never occurred."

"I'm ready to go home," Whelan said to CNN hours after the U.S. exchanged arms dealer Viktor Bout for Griner.

"I just need an airplane to come and get me."

The U.S. government says Russia was not willing to make a deal including Whelan ... claiming several attempts were made.