The Premier League club revealed on Tuesday that an agreement had been reached with Sevilla to bring the 22-year-old to Anfield and the Premier League club confirmed on Saturday that Moreno had signed a "long-term deal".
Moreno becomes Brendan Rodgers' eighth new signing of the new season, but will not feature in Liverpool's first game of the Premier League season against Southampton on Sunday as the deal was not completed in time.
He was omitted from Sevilla's starting line-up for Tuesday's UEFA Super Cup defeat to Real Madrid in Cardiff and cut an emotional figure as he seemingly said his farewells to team-mates and staff on the pitch.
But the defender felt the chance to move to Anfield was too good to pass up as he prepares for the next chapter of his career.
"In my view, Liverpool are the best club in England and I couldn't waste an opportunity to come and play for them." he told the club's official website.
"I spoke with Xabi Alonso, Pepe Reina and Alvaro Arbeloa and they told me this is a spectacular club whose fans follow it in a very special way.
"They all told me I can grow a lot here as a footballer as this is a massive club. I'm very happy to be joining a club as big as Liverpool. I hope to give my best and do all I can to help."
Moreno could come into contention for Liverpool's trip to champions Manchester City a week on Monday.
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