Kardashian West lobbied for the release of Alice Marie Johnson, who had been sentenced to life in prison for her minor role in the drug trade in 1996.
The model and TV personality tweeted a video of Johnson wearing her new Skims range of shapewear.
"Kim went to war for me, to fight for my freedom," said the mother-of-five.
"That's why I call her my war angel."
Johnson was sentenced by a US court in 1996 for a first-time, non-violent offence. She was unemployed and recently divorced when she became a "telephone mule", relaying messages between drug distributors and sellers.
"I participated in a drug conspiracy and I was wrong" she wrote in 2016. "It's hard to imagine that I have now served 20 years of my life sentence for that one mistake."
A petition for clemency was denied by the Obama administration in 2016, but her sentence was commuted by President Trump following a high-profile campaign led by Kardashian West and Mr Trump's son-in-law Jared Kushner.
She was released in June 2018. "Every moment in life is precious to me" Johnson says in the video tweeted to Kardashian West's 61.7 million followers.
Kardashian West's underwear line has already had a brush with controversy, and changed its name from Kimono Intimates in July following accusations of cultural appropriation.
Some Japanese people complained that the trademarked brand disrespected traditional clothing.