The 38-year-old plundered three goals inside the opening 20 minutes in his side's clash with Athletic Bilbao at Estadio Benito Villamarin.
His treble takes his LaLiga tally for the season to six goals – the same as his total from last season and the most he has scored in a domestic campaign since 2003-04.
Joaquin – capped 51 times by Spain – started his career at Betis before moving to Valencia in 2006. He then enjoyed spells at Malaga and Fiorentina before returning to Los Verdiblancos in 2015.
Real Madrid legend Alfredo Di Stefano had held the record as the oldest hat-trick scorer in the Spanish top-flight when he netted three times in a game at 37 years and 255 days of age.