Marseille were 5-0 up before Metz got on the scoresheet.
Morgan Sanson fired them in front after six minutes before Thauvin hit the first of his three with a volley two minutes later.
He got the better of Metz's Senegalese defender Fallou Diagne to score his second just before the interval.
That made it 3-0 at half-time against a Metz side who had only won two of their 10 games on the road road this season.
Valere Germain added the fourth just after the pause and Thauvin completed his hat-trick in the 56th minute following a slick pass from Dimitri Payet.
“Florian continues to be a decisive player,” said Marseille boss Rudi Garcia. “I want him to continue to work and to have an important contribution. He is a humble man. Praise can weaken men and I do not want him to be weaker."
Florent Mollet bagged a brace for the visitor’s with a goal either side of Kostas Mitroglou’s goal for Marseille after 75 minutes.
Ibrahima Niane’s strike in stoppage time for Metz brought some measure of respectability to the scoreline. But they appear doomed for the second division. They prop up the table in 20th place with 18 points after 24 games and are seven points off 17th which guarantees Ligue 1 action next season.
"When we score six goals in a home game, we can only be satisfied,” Garcia added. “We relaxed a bit at the end, but it was a great show."