Giroud fired France ahead after just 12 minutes at a rain-drenched Stade de France with Paul Pogba then heading in his first goal of the tournament to double the hosts’ lead.
Dimitri Payet struck two minutes before half-time to put France in complete control and Antoine Griezmann raced clear to add a fourth to round out France’s first-half blitz.
Iceland, who beat England 2-1 to reach the last eight, put on another never-say-die performance. Kolbeinn Sigthorsson and Birkir Bjarnason grabbed second-half consolation goals either side of Giroud’s second but Iceland were overpowerd by France’s domination.
Didier Deschamps’ men will take on Germany in Marseille on Thursday for a place in the final, while Iceland will return home as heroes after a remarkable run in their first appearance at a major tournament.
“Germany are the best team, there is no doubt about that, even if Italy gave them a few scares. But we are there in the final four,” said Deschamps.
Iceland, who had never beaten France in 11 attempts, kept faith with the side that shocked England, making European Championship history as the first team to name an unchanged line-up for their first five matches Samuel Umtiti became the first French outfield player to make his international debut during a major finals since 1966 as the Barcelona-bound defender replaced the suspended Adil Rami.
Moussa Sissoko filled in for N’Golo Kante, who was also serving a one-match ban, on the right side of midfield as Deschamps stuck with the 4-2-3-1 that helped France fight back to beat the Republic of Ireland in the last 16.
– Quick start –
Sluggish starts had been a hallmark of France matches in the tournament, but the hosts quickly seized control at the Stade de France with Hannes Halldorsson smothering Payet’s early effort at the second attempt.
It took less than a quarter hour for France to make the breakthrough though. Giroud raced onto Blaise Matuidi’s floated ball over the defense and drilled through the legs of Halldorsson.
Iceland, who fought back in style against England after conceding an early opener in their last 16 tie, could not replicate a similar response and fell further behind on 20 minutes.
Antoine Griezmann’s right-wing corner found a leaping Pogba who towered above Jon Dadi Bodvarsson to power in his first goal of the European Championship.
Jon Dadi Bodvarsson skied over after Aron Gunnarsson’s long throw caused trouble inside the French penalty area, but Les Bleus had wrapped up their place in the last four by half-time.
After Giroud knocked down a looping cross to Griezmann, the Atletico Madrid striker teed up Payet whose low drive whistled into the far corner two minutes before the break.
Griezmann tucked away France’s fourth on the stroke of the interval, running onto Giroud’s pass and lifting a deft chip over Halldorsson to move clear in the race for the Golden Boot with his fourth goal.
Sigthorsson, who netted Iceland’s winner against England, scored for the second game running after poking in Gylfi Sigurdsson’s cross on 56 minutes.
But France replied with Giroud beating Halldorsson to a Payet free-kick, and the Arsenal striker was withdrawn immediately having been booked earlier in the competition.
Iceland were rewarded for their perseverance though as they grabbed a second goal when Bjarnason headed in Ari Skulason’s cross on 84 minutes.