The 43 percent hike comes a year after a 40 percent increase.
"This change reflects an increase in costs associated with carrying bags, such as fuel and handling," the airline said in a statement.
The Chicago Tribune reported Monday that United's $100 fee matches Delta Air Lines, which also charges $100 for a second bag, though that fee is $80 with online check-in.
The larger fee is expected to give United "modest" returns, said Hunter Keay, an analyst with Wolfe Trahan & Co. in a report.
It is expected to increase revenue at United by $30 million per year, Keay said.