The 30-year-old will take the number three shirt at Anfield, which was recently left vacant by Brazil's Fabinho.
Endo joins Liverpool on a long-term contract, subject to international clearance and work permit, for a fee of around £16 million.
"I'm very happy right now and I'm so excited to join a big club in Liverpool," said the Japan star.
"It feels amazing and this is my dream. It has always (been) a dream to play (in the) Premier League and with one of the biggest clubs in the world. It's a dream come true for me."
Endo added: "It's going to be the first time for me that I play at Anfield. I can't wait to play at Anfield in front of Liverpool fans.
"I play as a number six and I'm like a bit more (of a) defensive player. I think I can help this club defensively and I will have good organisation in the middle."
Following several departures at Liverpool, including former captain Jordan Henderson, the Reds were in desperate need of midfield reinforcements.
Endo will plug one of these gaps, but Jurgen Klopp is expected to return to the transfer market for another defence-minded player.
Earlier this summer, Liverpool completed the signings of Alexis Mac Allister from Brighton and Hove Albion and Dominik Szoboszlai from Leipzig.
