McConaughey, 43, was forced to lose weight for his upcoming role in "The Dallas Buyer's Club" as Ron Woodroof, a mid-1980s AIDS patient suffering through experimental HIV cures.
The actor was seen Friday catching a flight from Los Angeles to New Orleans wearing tight jeans and a brown leather jacket, which he eventually removed to show his new overly svelte figure. People magazine has the photo, which you can view here.
"The Dallas Buyer's Club" began shooting in New Orleans on Sunday.
"Matthew starts shooting this weekend, so he's at his absolute lowest weight," a source told People magazine. "In a month, he'll begin to put it back on."
McConaughey began shedding pounds over the summer and appeared thinner while filming Martin Scorsese's "The Wolf of Wall Street."
“I should not look healthy by the time I'm doing this,” McConaughey told Larry King in July. “It takes a while for your body to understand that it has to feed off of itself and that you're not going to give it something else from the outside.”