Craig Clasen, 32, was out hunting yellow-fin tuna with three friends, who included a photographer and a film-maker, in the Gulf of Mexico.
While in Louisiana waters, south of the mouth of the Mississippi River, one of the friends was approached by a 3.5m tiger shark which, Clasen told journalists, seemed to be in attack mode.
"I positioned myself between Ryan [McInnis] and the shark and... noticed that the shark was getting tighter and tighter and just kept trying to get a back angle on us and behaving in an aggressive manner," he said.
"The shark made a roll and looked like it was going to charge us, so I just went ahead and took the conservative route and put a shaft through its gills.
"Cameron [Kirkconnell] and I have been around sharks for years and we all have a lot of experience with them but this encounter had a different feel to it."
After shooting the shark, Clasen was concerned to "finish the job" quickly and kill the creature "as humanely as possible", but the execution proved difficult.
"I shot it six times in the head with a spear and I wasn't having much luck," he said. A rope was tied around the shark's tail and it was towed behind the divers' boat in another failed attempt to kill it by drowning.
Clasen re-entered the sea to finally administer the coup de grace. "In the end we had to put a knife in its skull," he said.
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