Wade Seago told the BBC it had taken three shots from his revolver to kill the boar, which weighed 372kg (820lb).
He said the animal had terrified his daughter and disturbed his dog.
Feral pigs cause millions of dollars in damage across a swathe of southern US states because they damage crops and are believed to infect livestock.
"I didn't really have time to think about it," Mr Seago told the BBC. "I was confronted by an animal threatening my dog that looked like something out of a horror film."
Mr Seago works as a taxidermist in the southern Alabama town of Samson. He lives on a 100-acre property which abounds with wildlife including deer and raccoons.
He told local media that his encounter with the boar - known locally as a hog - began one morning last week when his daughter began screaming.
"I jumped up to see what was going on," he told al.com.
"I looked out the back window and saw nothing, so I ran to the front of the house where my daughter was looking out the window. I couldn't believe what I was seeing."
Mr Seago said his dog had trapped the huge boar in his front yard, about five yards from his front porch.
"It was not a good situation," he said.
Mr Seago said he had been concerned that the boar might injure or kill his dog, so he shot it with a .38 calibre revolver.
"By the time I got in a position to shoot, the hog was about 12 yards away," he said. "Cruiser was out of my line to the hog so I fired."
"It took three shots to take him down."
Hunters on private land in Alabama are allowed to kill feral pigs because of the threat they pose to indigenous wildlife.
A US taxidermist has described how he dramatically shot a huge wild boar that had wandered into his garden in the southern state of Alabama.