The 28-year-old will remain with the Terriers until June 2018, at which point the newly-promoted club will have the chance to make the move permanent.
Huddersfield boss David Wagner told the club's official website: "As everyone knows, we were very successful with our loans last season and now we have had the chance to bring another high-quality player to the club in this way.
"He was part of a club in Mainz that had a difficult season in 2016-17 and that is why we have this rare chance to take a player of his quality on loan. I trust and believe in his talent and potential.
"Jonas has all the skills that you want from a modern goalkeeper; he is a good shot-stopper, he is a real presence and he is good with the ball at his feet too. I also know he is also a good character; a real family man, who speaks fluent English. He will fit in perfectly with our squad."
Lossl joined Mainz from French side Guingamp in the summer of 2016.