U.S. officials said Thursday they've struck a deal with the foreign country ... agreeing to a prisoner swap to free the WNBA star.

It's a 1-for-1 trade ... with the U.S. slated to send over Russian arms dealer Viktor Bout -- AKA the "Merchant of Death" -- in the swap.

President Joe Biden revealed Thursday morning that he personally spoke with Griner after executing the trade, adding, "She is safe. She is on a plane. She is on her way home."

Griner has been detained in Russia since February -- after she was accused of bringing hashish oil in her luggage to a Moscow airport. In August, she was sentenced to nine years in prison. An appeal of that ruling was denied in October.

Last month, the 32-year-old had been moved to a prison in Mordovia ... where experts said she was facing homophobia, racism and 16-hour work days.

At the time, reps for Griner said she was "trying to stay strong."

At a news conference Thursday morning, with Griner's wife, Cherelle, at his side, Biden told the nation, "I'm glad to be able to say that Brittney is in good spirits. She's relieved to be finally heading home."

Cherelle added, with a big smile on her face, "Today my family is whole."

As for Paul Whelan, another American who the U.S. has classified as wrongfully detained in Russia, Biden confirmed Thursday they are still actively negotiating for his release.

"We are not giving up," he said. "We will never give up."