Barrasso is a Republican like Thomas, meaning the appointment will have no affect on the U.S. Senate's party breakdown.
Barrasso, an orthopedic surgeon from Casper, will serve until the beginning of 2009. A special election in November 2008 will decide who will finish Thomas' term in 2012.
Barrasso's appointment is effective immediately.
Thomas died June 4 while being treated for leukemia.
Freudenthal, a Democrat, chose Barrasso over Cheyenne attorney Tom Sansonetti and former state Treasurer Cynthia Lummis. The Wyoming Republican Central Committee had nominated the three after 31 people applied for the job.
"There are many factors that went into this decision, and it was the sum of these factors that led me to this choice," Freudenthal said in a statement. "While I don't intend to indulge the speculation on why I made this decision, I will say that I hope I made the right choice."