Weissmann is assuming the role previously designated to Dan Jennings, who was promoted to general manager.
Weissmann, 52, spent the past seven seasons with the A's, where he handled a variety of assignments. The past two years he served as a special assignment scout, and he was previously an area scout and a professional scout for Oakland.
Weissmann also was a scout for eight years with the Rays, and he spent four years with the Mariners.
Jennings and Marlins president of baseball operations Michael Hill worked together in the past when both were with Tampa Bay.
The Marlins are stressing drafting and developing players, and the A's are regarded among the best in the game in that regard.
As a player, Weissmann was right-handed pitcher drafted by the Cubs in 1979. He spent 11 seasons in the Minor Leagues before beginning his scouting career with the Padres in 1992.
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter @JoeFrisaro. This story was not subject to the approval of Major League Baseball or its clubs.