The Turku, Finland featherweight needed just eight seconds to flatten England Andy Ogle on Saturday night in Stockholm, Sweden.
Amirkhani came right out of the gate and blitzed Ogle with a flying knee. That sent Ogle backwards to the mat, when Amirkhani landed a serious of uncontested blows, causing the referee to step in and wave the fight off.
UFC color commentator Joe Rogan vigorously called the stoppage premature, but judging by reaction online, his opinion was in the distinct minority. If anything, it appeared the referee saved Ogle from taking several more uncontested blows.
The knockout was officially the fifth-fastest in UFC history. Duane "Bang" Ludwig's KO of Jonathan Goulet and Ryan Jimmo's finish of Anthony Perosh both clocked in at six seconds and co-hold the record.
Amirkhani (11-2) won his UFC debut and his second straight fight overall. Ogle (9-6) has lost four in a row.
"I told you, first round baby, first round," Amirkhani said. "I wrote it in a book. It was wonderful; I enjoy entertaining people. Next I want the UFC to give me some time to train and to beat the hell out of these guys."