The 27-year-old, who will spend the remainder of the season with the Spanish outfit, has vowed to score goals in La Liga.
"When I heard about the possibility of coming here, I didn't think twice. It's a very good opportunity for me, so I'm very grateful to Celta," he told reporters at his official unveiling.
"I'm going to work hard to help the club and give a lot of joy to the fans with the goals I'm going to score."
Welliton has been with Spartak since 2007.