The 25-year-old Spaniard helped fire Celta to safety last term with 12 goals in 34 league fixtures and has become Brendan Rodgers' third signing of the summer at Anfield.
Aspas told the Liverpool website: "It wasn't a tough decision at all. 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."
When asked about his strengths as a player and his favoured position, Aspas said: "I'm a quick and skilful player. 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."