Lahtela, the silver medallist at Nagano, produced two four-element jumps and a super-fast pace in the final to beat novice Travis Mayer of the United States into second.
The Olympic gold completes his sweep of top honours that includes top World Cup finishes for two years from 1998, and his World championship crown in 1999.
Mayer, who only appeared at the top level for the first time last year had led the qualifying.
But the 19-year-old had to settle for silver with the bronze going to Richard Gay of France.
Nevertheless Mayer, who only qualified for the Games in December, was overjoyed by his performance.
"To make the Olympics was way beyond my expectations," he said.
"I thought if I skied my best for a couple of runs that would be a great day.
"But to end up second behind one of the greatest moguls skiers ever, it's the ultimate Olympic experience."
American Jonny Moseley, the reigning champion, again unveiled his new "dinner roll" jump - a spectacular off-axis 720-degree spin - which delighted the crowd.
But he failed to fully convince the judges as he finished just outside the medals.
The event sees competitors negotiate a course of bumps, known as moguls, at high speed while performing two jumps off ramps.
Scores are given for speed, jumps and technique. Lahtela finished top of sixteen skiers who qualified from the morning round.