Wehrlein believes his performance on Sunday will be compromised by a lack of fitness and training, and therefore volunteered to stand down.
The German was injured in a heavy crash in January's Race of Champions, which left him unable to train and forced him to sit out the first pre-season test of the year. On that occasion Giovinazzi - runner-up in the newly rebranded F2 championship last year - was again called up to replace Wehrlein.
"My fitness level is not as it should be for a full race distance because of my training deficit," Wehrlein explained in a team statement.
"I explained the situation to the team yesterday evening. Therefore, the Sauber F1 Team has decided not to take any risks. It is a pity, but the best decision for the team."
Team principal Monisha Kaltenborn insisted Wehrlein will be back in the car in China, adding: "We have great respect of Pascal's openness and professionalism. This decision was definitely not an easy one for him, it underlines his qualities as a team player.
"The focus is now on his fitness level, and in such a situation we do not take any unnecessary risks. Pascal will be in China as planned."
Giovinazzi, also test and reserve driver at Ferrari, is therefore set to be the second rookie on the grid in Melbourne, alongside Williams' Lance Stroll.
He will be the 84th Italian to start a Grand Prix - and the first since Jarno Trulli and Tonio Liuzzi took part in the 2011 Brazilian Grand Prix.