The multi-year deal includes strategic, commercial and technological cooperation, enhancing an arrangement that already included use of the latest specification Ferrari engine.
Leclerc, who turned 20 in October and is from Monaco, became the youngest F2 champion last season, dominating the title race and eventually winning by 72 points.
Giovinazzi, who served as Ferrari's third driver in 2017, has two Sauber race outings to his name, the Italian having stood in for the injured Wehrlein at this season's two opening rounds, with a best finish of 12th in Australia.
Alfa Romeo has joined up with the Sauber team for a return to Formula One after an absence of more than 30 years.
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The field also includes the European Tour's Race to Dubai victor Tommy Fleetwood and the red-hot Justin Rose, the 2016 Olympic gold medallist.
Leclerc has risen rapidly through the single-seater ranks, winning the GP3 title a year ago before his F1 championship triumph this season.
"Formula One is the natural reference point for a marque like this", he said.
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Charles Leclerc is definitely a driver to keep your eye on even though he is only set to drive for an uncompetitive Sauber team in his rookie season next year.
Besides the one remaining seat at Williams alongside Lance Stroll, Sauber was still yet to confirm either of its 2018 drivers, with three candidates being in contention.