The 25-year-old became Brendan Rodgers' third summer arrival once personal issues with his agents were sorted over the weekend.
"It wasn't a tough decision at all. I'm really happy to have come to a big club like this one," he said.
"Having weighed up all my options for this window, a massive club like Liverpool was always my first choice."
Aspas scored 12 goals in 34 league matches as Celta Vigo avoided relegation on the last day of the season.
By then Liverpool already had a deal lined up, having agreed a fee in the region of £7.7million.
"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," he told liverpoolfc.com.
"I am a quick and skilful player. Together we will achieve a lot of success."
Aspas will join Manchester City defender Kolo Toure and Sevilla midfielder Luis Alberto in being officially registered, subject to international clearance, when the transfer window formally opens.
Sunderland goalkeeper Simon Mignolet is set to follow this week as he is due at the club's Melwood training ground to discuss personal terms and undergo a medical after the two clubs agreed a fee, believed to be worth a total of £11million-plus, on Friday.
That has put the future of long-time first-choice Jose Reina in doubt as a speculated move to Barcelona has not yet materialised and the Catalans' number one Victor Valdes insists he will see out the remaining year of his contract.
But Reina, currently on international duty with Spain, does not believe his position has altered.
"It has not changed a bit," he told reporters after Spain's 3-0 win over Nigeria at the Confederations Cup in Brazil.
"I have said it many times, I'm very comfortable and very happy at Liverpool.
"Every team needs competitors, and when it's positive, it is more than welcome.
"If it is finally done, I'll welcome him a lot and anything he needs, I'm here."