Colorado Rockies outfielder Jorge Piedra was suspended for 10 days on Monday.
The suspension begins with the Rockies game at Arizona on Monday, the commissioner's office said.
Piedra, who was recalled from Triple-A Colorado Springs of the Pacific Coast League on Wednesday and sent back down the next day, is the second player to be publicly identified under the major leagues' tougher rules. Tampa Bay outfielder Alex Sanchez was suspended for 10 days last Monday.
Under the policy that took effect last month, steroids and other performance-enchancing substances are the only drugs to draw a 10-day suspension. MLB officials and the players' union agreed they would not disclose the exact substance for which a player tests positive.
The Rockies issued a statement Monday calling the situation "unfortunate."
"As an organization we have, and will continue to support Major League Baseball and its drug testing policies," team president Keli McGregor said.
At last month's congressional hearings on steroid use in baseball, lawmakers were critical of the plan's penalties, including a provision allowing for fines instead of suspensions. A first offense could cost a player $10,000 instead of 10 days out from a six-month season.
Using most steroids without a doctor's prescription for medical purposes is illegal. Baseball banned steroids in September 2002 and began testing for them with penalties in 2004.