Emery won a domestic treble with PSG in 2017-18, but came under fire after failing to advance his side past the Champions League last-16 phase.
The Spaniard confirmed he would be stepping down at the end of the season in April, and Tuchel is the man PSG have chosen to lead them after the World Cup.
"Paris Saint-Germain are happy to announce the arrival of Thomas Tuchel as first-team coach," the club revealed in a statement.
"The German coach has signed a two-year contract with the French champions."
Speaking to the club's official website, Tuchel affirmed that he was relishing the challenge of occupying the hotseat at Parc des Princes.
"It is with great joy, pride and ambition that I join this huge club in world football that is Paris Saint-Germain," he said.
"I hope to work with these great players, all of which are among the best on the planet.
"I will do everything possible with the help of my team to help the team reach the top levels of international football. There is an extraordinary potential in Paris and this is the most exciting challenge of my career.
"I am also keen to see Parc des Princes, a legendary football stadium with a fantastic atmosphere."
PSG president Nasser Al-Khelaifi, meanwhile, praised Tuchel's propensity for dynamic attacking football and his strength of character in welcoming the German to the club.
"I am very happy to welcome Thomas Tuchel as the new PSG coach," he said.
"Thomas is one of the most competitive coaches to emerge at the top level in recent years. He has strongly absorbed the principles of efficiency and spectacle that have always given German football great strength, especially at international level.
"His ambitious personality, his strong taste for playing a flowing game and his strength of character are part of the style we have always sought at PSG. This is the style that fans of our club have always wanted and admired."
An impressive two-year spell at Signal Iduna Park came to an end in 2017, but that tenure and a brilliant stint with Mainz have cemented his reputation as one of the sport's most talented young coaches.