Wall has long maintained that the TransCanada Corp. pipeline that would run through Alberta and Saskatchewan and then south into the U.S. would be good for the province's oil industry.
Wall and the governors, all of them Republicans, say the Keystone XL would generate thousands of jobs on both sides of the border and should be approved.
A year ago, amid opposition from environmentalists, citizens and politicians, Obama rejected the pipeline application.
However, he's expected to make a decision about a revised application by spring.
"Mr. President, we consider the Keystone XL Pipeline fundamentally important to the future economic prosperity of both the United States and Canada," the letter says. "We strongly urge you to issue a Presidential Permit and act swiftly to approve the Keystone XL pipeline."
Wall was the only Canadian premier who signed the letter.