The two sides are finalizing the details of what will be a three-year deal with an option for a fourth, a person familiar with the situation told the Washington Post.
Martinez has served as Joe Maddon's bench coach in Chicago and Tampa Bay (2008-14) for a decade but has never been a manager. When Maddon became the manager of the Cubs before the 2015 season, Martinez followed him to Chicago. A year later, the Cubs won the 2016 World Series title for the team's first championship in 108 years.
The 53-year-old Martinez was considered the front-runner to replace Dusty Baker, who was fired last week after two seasons.
Martinez, a former outfielder who played for nine different teams during his 16 years in the majors, will become the seventh full-time manager for the Nationals since they moved from Montreal before the 2005 season.
The Nationals received permission to discuss the job with Martinez on Monday and he interviewed in Washington on Thursday, according to the Post.