United says the fee affects customers who buy tickets beginning Friday for domestic flights from Aug. 18 or later.
Also, the Chicago-based carrier is increasing the fee to check three or more bags, overweight bags or items that require special handling to $125 from $100, or to $250 from $200, depending on the item.
The fees reflect a struggling airline industry passing the costs of high fuel prices to passengers.
Airline expert Tom Parsons says, as of July 1, Southwest Airlines will be the only U.S. carrier that permits two checked bags for free.
