Howard Johnson, who had been serving in that role, was reassigned to the minor leagues.
"Watching the club it had become apparent to me that we had reached the point that a change was necessary," said Mariners general manager Jack Zduriencik. "I appreciate Howard Johnson's professionalism and work ethic, but we simply were not getting the results that we expect and require.
"I have had the opportunity to observe Edgar over the past several years at Spring Training and working with our young hitters in the minor leagues. He is passionate and knowledgeable about hitting and understands the work that needs to be done to be successful as a big league hitter. His resume, desire to contribute at this time and ability to convey complex information about hitting is what drew us to this decision."
The Mariners entered Saturday's action ranked dead-last in the majors in batting average and 28th in the majors in runs scored.
Martinez has spent the last several seasons working as a guest hitting instructor at Spring Training, and had an extended schedule during the season this year working in Seattle's minor league system.
The 52-year-old Martinez played his entire 18-year career with the Mariners. In 2,055 career games, he posted a .312 batting average with 514 doubles, 15 triples, 309 home runs and 1,261 RBI. He won American League batting titles in 1992 and 1995.