Players began making decisions Monday whether to participate during the ongoing coronavirus pandemic.
Zimmerman, 35, previously told the Associated Press he was on the fence about playing.
"I have a 3-week-old baby," Zimmerman told the AP. "My mother has multiple sclerosis and is super high-risk; if I end up playing, I can pretty much throw out the idea of seeing her until weeks after the season is over."
Monday, Zimmerman released a statement saying he will not retire -- yet.
"After a great deal of thought and given my family circumstances -- three young children, including a newborn, and a mother at high risk -- I have decided not to participate in the 2020 season.
"Everyone knows how much it means to me to be part of a team, and I will miss that camaraderie dearly this year. Of course I would love to pursue back-to-back titles. I cannot speak for anyone else, but given the unusual nature of the season, this is the best decision for me and my family, and I truly appreciate the organization's understanding and support.
"To be clear, I am not retiring at this time. I have not decided on my future in baseball past 2020. But this year, I'll be staying safe at home and pulling as hard as anyone for the guys to defend our championship."
Both of Ross' parents are in the medical field. His father is a pediatrician. His mother is an emergency room nurse.
Ross was expected to be part of the competition for the fifth starter spot in the Nationals' rotation this season.
"Ryan Zimmerman and Joe Ross have decided not to participate in the 2020 season for the personal health and safety of themselves and their loved ones," Mike Rizzo said in a statement. "We are one hundred percent supportive of their decision to not play this year. We will miss their presence in the clubhouse and their contributions on the field."
Both of their contracts are believed to be pushed to next season because neither will receive service time or prorated pay in 2020.
The Nationals' 60-player pool is down to 58. They can replace Zimmerman and Ross from within or sign other players to add to the pool.
The Athletic and MASN were first to report the decisions.
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Ryan Zimmerman, Joe Ross opt out of 2020 MLB season originally appeared on NBC Sports Washington