Melisa Reidy-Russell wrote that she suffered physical and emotional abuse throughout their two-year marriage and that the mistreatment severely affected her.
"With the new details revealed ... by Ms. Russell, Mr. Russell has been placed on Administrative Leave in accordance with the Joint MLB-MLBPA Domestic Violence Policy," MLB said in a statement Friday. "We are hopeful that this new information will allow us to complete the investigation as promptly as possible."
Russell responded in a statement through the Major League Baseball Players Association.
"These allegations are completely false," Russell wrote. "I made that clear to Major League Baseball last year and reiterated it to the Cubs today. I'm confident any full and fair investigation will fully exonerate me. The protection of my children is foremost in my mind so I will have no further comment."
Previous allegations by a friend of Reidy-Russell triggered an MLB investigation into Russell last year that remains open.
"The first time I was physically mistreated by my spouse, I was in shock," Reidy-Russell wrote. "I couldn't wrap my head around what just happened ... Why did he get so angry? What did I do for him to want to put his hands on me? Of course I forgave him & assumed it would never happen again. I just thought he had let his emotions get the best of him, he loves me and he's sorry. I was deeply hurt that he could even be capable of this behavior towards me, I couldn't understand how the man I was so in love with, the FATHER of my child, the man I married just a few months ago could show such aggression towards me."
Reidy-Russell wrote that emotional and verbal abuse became an "everyday thing" in their relationship.
"Being blamed for just about anything that went wrong, name calling, intimidating me with personal force, manipulating me to think I was the problem, destroying my personal things, threatening me to 'send' me & our son home to my parents as if I was privileged to be living in our home," she wrote. "Basically, I felt like I was nothing, a nobody & I was nothing without him, & I couldn't do anything without him."
Chicago Police spokesman Anthony Guglielmi told The Associated Press that the department has not launched a criminal investigation, "at this point," but detectives will look at the allegations "further and have already reached out to the Cubs."
"Obviously, we take it very, very seriously," Cubs owner Tom Ricketts added. "The place it's at right now, it's in the league's court to do the investigation."
Cubs president of baseball operations Theo Epstein said he again asked Russell on Friday if the allegations were true.
"We questioned him on it," Epstein said. "He reaffirmed his stance that he did not do what he is accused of having done."
Manager Joe Maddon said he was surprised by the new accusations.
"I have not yet spoken to Addy about it," Maddon said Friday. "From our perspective, it seemed like it was done, but apparently it's not."
MLB opened its probe in June 2017 after Reidy-Russell accused Russell of cheating in an Instagram post and a friend commented with an allegation of abuse by the Cubs shortstop.
Russell denied the accusation at the time, and Reidy-Russell declined to cooperate with baseball investigators. MLB never formally announced an update on the investigation.
"Last year, this was a third-party accusation on social media, and now this is a direct accusation from the potential victim," Epstein said Friday. "The situation has changed. He was not placed on administrative leave last year. We think this step is appropriate in light of the post.
"It would have sent the wrong message to have Addison wearing a Chicago Cubs uniform this morning, so we fully support him being placed on administrative leave."
MLB and the Cubs both issued statements Friday saying they take allegations of domestic violence seriously.
Epstein said he was aware that the investigation was ongoing and that he has been in touch with MLB's investigative unit.
"All parties have an obligation to get to a just and fair resolution. If that includes discipline, so be it. If it doesn't, so be it," Epstein said. "The important thing here is that justice and fairness is ultimately found. Timing isn't ideal, but it doesn't matter."
Reidy-Russell, who said she dealt with depression caused by Russell's abuse, wrote about the effect leaving her husband had on her overall well-being.
"The way I was treated and the way he made me feel about myself, tore me down to nothing. I was so proud of myself for not being afraid anymore, I was free to make my own choices and I was free to live for MYSELF," she wrote. "I was taking back my life. It took months for night terrors to stop, to not have panic attacks 3 times a week, to look in the mirror and not feel worthless."
The couple had a son in August 2015, and they were married in January 2016. Their divorce was finalized Aug. 30.
Information from The Associated Press was included in this report.