Jenny Potter and Andrea Kilbourne also scored, and goaltender Sara DeCosta stopped 21 shots for her third Olympic shutout.
The US's 34th consecutive victory set up a semi-final clash with Sweden, who lost 11-0 to seven-time world champion and 1998 silver medallist Canada in the last Group A game on Saturday.
Darwitz, an 18-year-old first-time Olympian, now has six goals in three games, taking over the Olympic scoring lead from team-mate Cammi Granato.
US captain Granato, who assisted on Darwitz's first goal, had also notched a hat-trick against China on Thursday.
Final Group B table
1. USA 6pts
2. Finland 4pts
4. China 1pt
Although Canada are the only team to have ever beaten the US in international competition, the margin of this victory was something of a surprise.
Finland have twice held the US to a draw in the past and have played well in the group stages.
Their 21 shots were more than the American goaltenders had faced in their previous two games combined.
But DeCosta was strong again in the US net and with Darwitz in such form there was no stopping the home side.
Canada in control
Of her continued success in front of goal, Darwitz said: "It's amazing. You dream of scoring a goal in the Olympics but to get three in one game is surreal.
"The puck just bounced my way. It's very thrilling to get a hat-trick and scoring goals but it's even better winning the game and moving on in the tournament."
The other match in Group B, the dead game between China and Germany, ended 5-5.
But in Group A, Jennifer Botterill scored two goals in Canada's 11-0 rout of Sweden.
Canada did not concede a single goal in round-robin programme and outshot its three opponents with a combined 25-0 score.
Botterill opened scoring midway through the first period and tipped in 3-0 halfway through the second with the Swedes short-handed.