Costas' addition, effective and influential immediately, was announced Tuesday morning by the Network.
Costas will not delay in impacting the month-old MLB Network: He is scheduled to conduct an hour-long interview later Tuesday with former Yankees manager Joe Torre for broadcast Thursday night.
As part of his multiyear deal, Costas, formerly affiliated with HBO, will make his exclusive cable home on MLB Network.
The host of NBC's recent Super Bowl telecast will host special original MLB Network programming with an emphasis on the game's history and its most significant figures and topics, and serve as the lead play-by-play commentator for a select number of MLB Network's slate of 26 live regular-season games.
Costas will maintain all his current duties at NBC, including his role as host of that network's Olympic and NFL coverage.
Commissioner Bud Selig welcomed Costas into the MLB Network fold as "one of the preeminent sports broadcasters of his generation."
"Bob also cares very deeply about the game of baseball," Selig added. "We're glad to reunite him with the game he loves most and look forward to the exciting television he and MLB Network will create together in the years to come."
Thursday's interview of Torre, whose controversial book, "The Yankee Years," was also released on Tuesday, will not be Costas' debut on the MLB Network. He had been instrumental in the network's Jan. 1 launch with interviews of the battery of Don Larsen and Yogi Berra during a special broadcast of Larsen's perfect game in the 1956 World Series.
"Bob showed us all on Jan. 1 how his presence can truly elevate a broadcast, and we plan to tap into his unique abilities and perspective to build more high-caliber programming in the future," said Tony Petitti, president and chief executive officer of MLB Network. "In addition to the studio show, we're also looking forward to Bob's return to the broadcast booth, where he's been part of so many memorable moments in baseball over the years."
Costas is an eight-time honoree as Sportscaster of the Year by the National Sportswriters and Sportscasters Association.
He is the latest addition to MLB Network's on-air talent, joining Matt Vasgersian, Victor Rojas and Greg Amsinger, studio analysts Sean Casey, Barry Larkin, Al Leiter, Joe Magrane, Dan Plesac, Harold Reynolds and Mitch Williams, reporters Trenni Kusnierek and Hazel Mae and baseball insiders Jon Heyman and Tom Verducci.
Tom Singer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.