"Today, I have submitted my resignation from the job I love -- managing the Rangers -- in order to devote my full attention to addressing an off-the-field personal matter," Washington said in a statement. "As painful as it is, stepping away from the game is what's best for me and my family."
The Rangers have the worst record in baseball this season at 53-88 and are 32 games back in the AL West after Friday night, when they fell 7-5 to the Seattle Marinersfor their seventh straight loss.
General manager Jon Daniels said Washington's resignation is "not drug-related."
"We don't want to talk about any of the specifics," Daniels said. "Ron's given us permission to acknowledge this was not drug-related."
According to a source, Washington's health wasn't a factor in the decision.
Washington didn't respond to text messages from ESPN seeking comment. He did say in a text to USA Today Sports: "I'll be back! Need some time!''
In 2009, Washington tested positive for cocaine and was subjected to random drug testing. Washington begged for forgiveness and even offered to resign, but then-team president Nolan Ryan stuck with him, signing him to a two-year contract extension in 2012.
Beset by injuries, the Rangers have used 40 pitchers among the 63 players to see action this season -- both major league records for a season.
"This has been a difficult season for the team for a variety of reasons," Daniels said. "But it was very clear throughout the organization, both publicly and privately with Ron he was coming back. We were planning on him coming back as our manager for 2015. The bottom line is you don't have a season like we had without a number of things going wrong. First and foremost with decisions that I've made, and that's where the accountability counts. And that's not what this decision or outcome is about. It's not an on-field matter."
Washington, 62, had a 664-611 record in his eight seasons with the Rangers, guiding the franchise to its first two World Series appearances (2010, '11). He is the franchise's leader in regular-season wins and games managed.
"This is in no way related to the disappointing performance of the team this season," Washington said in the statement. "We were already discussing 2015 and looking forward to getting the Rangers back to postseason contention.
"I deeply regret that I've let down the Rangers organization and our great fans. Over the past eight seasons, it's been a privilege to be part of some of the best years in club history, and I will always be grateful for the opportunities I've had here, and for the great management, players and coaches who have made our time here a success."
Daniels said the club knew about Washington's situation for weeks and that a final decision about him leaving was made Friday morning.
A leave of absence was discussed with Washington but ultimately leaving the club now was best for everyone, he decided.
On the white board inside the Rangers clubhouse was a sign about a 3 p.m. meeting. Washington then announced to the club at that meeting that he was resigning.
Bench coach Tim Bogar will be interim manager for the remainder of the season.
"It's obviously not how you want to become a manager for the first time, especially when you take over for a really good friend," said Bogar, who was choked up during his discussion with reporters. "I've known Wash for a long time and not only as a coach; he coached me in Triple-A and he basically taught me how to get to the big league. And as a colleague and as his bench coach, but most of all I feel like I'm his friend; that's what I'm going to miss most is our friendship together and the times we spent on the bench."
Added Daniels in a statement: "While we are disappointed, we accept Ron's decision and are grateful for his many contributions to the Rangers organization over the last eight years. This has certainly been a difficult season for our major league club in terms of on-field performance, but we were looking forward to moving ahead with Ron as our manager in 2015."
The Rangers were 18-16 in Washington's three postseason appearances.
As a player, Washington was a skinny middle infielder who had more than twice as many games in the minors than the majors in 20 seasons as a pro. He then spent four years as a minor league coach before 11 seasons as an assistant in Oakland, the last 10 as the third-base coach before the Rangers hired him after the '06 season.
Washington always said the lessons he learned growing up in the big leagues were ones he wanted to pass on to his players. He was an emotional leader for the club, and several players expressed shocked at the sudden departure.
"The guy is like a father to me and honestly I felt like I lost my dad," left-hander Derek Holland said. "He taught me a lot both on and off the field and like I said, he meant a lot to me. It's going to be hard not to see Wash, especially walking down and going to the weight room and seeing him and goof off and making goofy comments to him."Friday night's loss was the debut as big league manager for Bogar, who was in his first season as Washington's bench coach. He was a minor league manager for five seasons.
"I don't think fans here in this area have seen anybody in that seat for eight years," Bogar said. "And I've got to do it after an icon, so it's a little bittersweet."
The Associated Press contributed to this report.
Washington's Resignation Not Drug-Related
Texas Rangers general manager Jon Daniels says that Ron Washington's resignation as manager is not drug-related.
Ron Washington Resigns
John Kruk reacts to the news that Ron Washington has resigned as manager of the Texas Rangers.