The 31-year-old, who spent two years with St Mirren a decade ago, has more than 300 club appearances and 52 caps.

Celtic have an option to buy 23-year-old centre-back Jenz.

The former Fulham youth moved to Lorient on a five-year deal from Swiss side Lausanne-Sport last summer and went on to make 18 appearances, 17 of them starts, as the club finished 16th in Ligue 1.

He moves to Glasgow alongside Mooy, who was reportedly wanted by several Championship clubs having previously turned out for Huddersfield and Brighton.

Postecoglou said: "I'm delighted we've been able to bring in both Aaron and Moritz and I'm sure they'll be great additions to our squad.

"I obviously know Aaron from my time coaching the Australian national team, and he brings with him a wealth of experience at both club and international level.

"Moritz is a player we've been aware for a while now, and I'm pleased that we've been able to sign him. They've already had the chance to meet the rest of the squad, and I'm sure they'll settle in very quickly."

Celtic have also signed Alexandro Bernabei and Benjamin Siegrist this summer as well as sealing permanent deals for Cameron Carter-Vickers, Jota and Daizen Maeda, who had been on loan.