"The deal is subject to the footballer passing the relevant medical tests," Valencia said in a statement on their website (www.valenciacf.com).
"It was a tough job sealing the accord because Nelson is a highly valued player at Rubin," sporting director Braulio Vazquez added.
"He is a very important player for us because he is a forward who fits very well with the players we have," he said.
"Nelson Valdez has strength, commitment, speed and gets forward very well. He will also give us goals. We have a lot of confidence in him."
Valencia finished third in La Liga last season, earning a place in the Champions League.
They begin the 2012-13 La Liga campaign on Sunday at champions Real Madrid. (Reporting by Iain Rogers, editing by Matt Barker)
- Nelson Valdez