The Spanish striker, who scored 72 league goals in four years with the recently relegated club, joins Ole Gunnar Solskjaer's men as they prepare for life in the Championship
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The 32-year-old spent four years at the club, the last two of which were in the top division, and scored 72 league goals after joining from Levante.
His tally of 15 in 2013-14 could not save Valladolid from relegation back to the Segunda Division and he joins a Cardiff side who themselves dropped back into the Championship after just one year in the Premier League.
"I am very thankful for the warm welcome I have received here at Cardiff City and am very excited for my new challenge in Wales," Guerra told the club's official website.
"Hopefully I can keep up the level that I have been playing at in Spain for the last few seasons and that it will be a successful time for me in Cardiff, with both the club and the supporters."