"It wasn't a tough decision at all," Aspas told Liverpoolfc.com. "I'm really happy to have come to a big club like this one.
"Having weighed up all my options for this window, a massive club like Liverpool was always my first choice."
Aspas fired 12 goals in 34 league fixtures during 2012-13 to help Celta avoid relegation from the Spanish top flight.
"I am a quick and skilful player," he explained when asked about his key attributes, before sending a message to Liverpool fans: "Together we will achieve a lot of success.
"I've not had the chance to talk with the manager at any length just yet but I think that good players can slot into any role and not just one set position."