After a three-and-a-half years at Stamford Bridge, Emerson will feature for the first time in the French Ligue 1.
Emerson's stint with Chelsea saw him winning the Champions League, Europa League, European Super Cup and FA Cup.
The former AS Roma defender was also part of the Italy's team that won the Euro 2020 in July, making four appearances in the competition and also started in the final against England.
Following the injury of Leonardo Spinazzola in the quarter-final, Emerson became the first choice at the left-back for the Azzurri.
Lyon have a confirmed buy-option, as high as €15 million, included.
Lyon finishing fourth in Ligue 1 last season have gotten off to a rather poor start and are winless in their first two games of the season as they suffered a 3-0 defeat against Angers SCO last weekend.
Emerson recorded his 71st appearance for Chelsea in last Saturday's Premier League opener against Crystal Palace, coming off the bench to replace Marcos Alonso in the 86th minute.
The 27-year-old joined Chelsea from AS Roma for an initial £17.6 million, signing a four-and-half-year deal with the Blues.
Emerson, who was signed as a back-up to Marcos Alonso dropped to the pecking order at Stamford Bridge following the arrival of Ben Chill from Leicester City.
The Italian defender will compete in the Europa League with Lyon and is also expected to make his Ligue 1 debut when Lyon face newcomers Clermont on Sunday.
