The report said Plank brought Ruhle on his personal private jet and took her business advice, citing people familiar with the matter. In some cases, advice on topics like how to manage his relationship with President Donald Trump, superseded that of some executives, the Journal said, citing sources.
Employees viewed her input as unusual, because "many people believed she was romantically involved with Mr. Plank," the paper reported.
A person familiar with the situation told CNBC that the relationship between Ruhle and Plank had no impact on Under Armour business. The person requested anonymity due to the sensitivities surrounding the matter.
The Journal report cited Under Armour's senior vice president of communications, Kelley McCormick, as saying "Mr. Plank and Ms. Ruhle are friends." She also added, "The idea that Mr. Plank uniquely listens to any one individual is absurd."
The company's board became aware of Plank's relationship with Ruhle after it discovered emails that "showed an intimate relationship between them," the paper said, citing sources. The board asked Plank about his relationship with Ruhle and were told it "was a private matter and that no company funds were spent."
A person familiar with the situation reiterated to CNBC that no Under Armour money was spent pertaining to Plank's relationship with Ruhle.
The news comes at an inopportune time for the sportswear company, which has been looking to turnaround its company culture. Plank has said he wants to build a "diverse" and "inclusive" company, following reports the company had been letting its employees charge visits to strip clubs on their corporate cards.
"All these things that are coming out, it's difficult," Plank told CNBC's Sara Eisen in a December interview. "You know, we are in our 14th year as a public company. That much time in the arena means that at some point you are going to have to retool the machine."
MSNBC declined to comment.
An Under Armour spokeswoman referred CNBC to her statements to the Wall Street Journal.
Shares of Under Armour dipped by nearly 2 percent Thursday.
Disclosure: MSNBC and CNBC are divisions of NBCUniversal.