The club also have an option to extend his deal for a further 12 months.
Former Spain international Hernandez has made 15 Championship appearances since joining in August.
The 31-year-old previously played alongside - and under - Leeds head coach Garry Monk at Swansea between August 2012 and February 2014.
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Leeds United have signed on-loan forward Pablo Hernandez from Qatari side Al-Arabi until the end of the season for an undisclosed fee.