Mauricio Pochettino on his first day at Cobham training ground
The Blues have won 21 trophies since 2004, including five Premier Leagues and two Champions Leagues.
But they sacked two managers last season and finished 12th.
"In the last 10, 12, 15 years, Chelsea is the greatest team in England," former Southampton and Tottenham Hotspur boss Pochettino said.
"I know very well the Premier League and what the culture of Chelsea means. I think our fans are excited to again be on the road of trying to win."
The 51-year-old Argentine won three trophies in his time as Paris St-Germain manager, including the Ligue 1 title in 2021-22.
He failed to win anything in his previous six-year spell in England, although did reach the 2019 Champions League final with Spurs.
"We are excited to work with a very young team, with a different approach than in the past," he said.
"But I think we all need to understand that we have to work really hard and create a very good atmosphere at the training ground to build success for the team in the next few years.
"For me, it is a pleasure and honour to now be involved with Chelsea Football Club. We are so excited, and I know Chelsea very well, it is one of the greatest clubs in the world. So of course, it was easy for us to make the decision to move here."
New owner Todd Boehly sacked Thomas Tuchel and Graham Potter in his first season, with the club spending about £288m in January, which was more than the combined total of all clubs in the Bundesliga, La Liga, Serie A and Ligue 1.
Pochettino has taken over a squad that has seen more change this summer, with Kalidou Koulibaly, Edouard Mendy and N'Golo Kante all leaving to join teams in Saudi Arabia, England midfielder Mason Mount agreeing to join Manchester United, Ruben Loftus-Cheek moving to AC Milan and Mateo Kovacic signing for champions Manchester City.
France striker Christopher Nkunku has arrived fromRB Leipzig for £52m and Senegal forward Nicolas Jacksonjoined on an eight-year contract from Villarreal.
Pochettino however said a "very clear project" is now in place which may take time but Chelsea still need to "be at the top".
He added: "From the beginning, the owners and sporting directors were very clear about the project and we were very excited to join them in this project.
"We are here to try to help the club and the fans. In the end, the most important thing in football is for them to be happy and to feel proud of us and in the way we approach games. The players need to know that.
"Of course, there are always up and downs in the history of football, but Chelsea is a club that it is impossible to have these up and downs.
"We need to be sure that we bring what the club needs to be at the top because the history of the club is to be at the top."
-- Courtesy of BBC Sport