Fallin, a Republican, told reporters that she asked the president in a meeting at the White House about his timeline for making a final call on the controversial pipeline that would bring oil from Canada to the Gulf Coast.
The Oklahoma leader said Obama replied that “one way or another, in a couple of months.”
The State Department is now reviewing the project, but a judge last week ruled that a Nebraska law allowing the pipeline to be built in the state was unconstitutional.
Environmentalists are also pressuring the president to block Keystone, but a recent State Department report found that construction of the pipeline would not lead to a significant increase in carbon emissions.
White House press secretary Jay Carney later Monday would not confirm nor deny Fallin's characterization of her conversation with the president.
Democrats, however, bristled at that description.
“That’s the most insane statement I’ve ever heard,” countered Gov. Daniel Malloy, D-Conn.
The chief executives, including a handful of possible 2016 presidential candidates, were at the White House for a meeting of the National Governors Association.
This story was published at 1:24 p.m. and has been updated.