The 29-year-old Ben Arfa, a rare talent who has struggled for consistency over the years, scored 17 goals for Nice last season to help them a finish a surprise fourth in the Ligue 1 standings.
The most prolific season of his erratic career earned Ben Arfa a recall by France coach Didier Deschamps in the build-up to the European Championship in France.
He was among eight reserves named by Deschamps in his provisional squad for the tournament but did not feature in the final 23-man list.
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After making his name at Olympique Lyon, Ben Arfa played for Olympique Marseille and Newcastle United before a brief spell with Hull City. He joined Nice last year.
Born in Clamart, outside Paris, he had said several times that he would love to play for PSG.
"Signing with Paris St Germain is a very happy and a proud moment... I will do my utmost to represent my new team to the best of my abilities and to make our supporters proud," Ben Arfa said through a club statement.
Emery, who guided Spanish club Sevilla to a third consecutive Europa League title last season, replaced Laurent Blanc, who parted company with PSG after three seasons in charge.
PSG achieved their second successive domestic treble by winning Ligue 1, the French Cup and the League Cup, but they lost in the Champions League quarter-finals for the fourth successive season.
(Writing by Richard Lough and Patrick Vignal, editing by Ed Osmond)
