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.