The winger joins from Valencia for a club record £5.55 million.
Hernandez told Swanseacity.net: "I'm very happy to become a Swansea City player. It was my target as soon as I knew there was interest. There were offers from other clubs, but this was the only club I wanted to join.
"I want to thank the club and Michael Laudrup for making me feel like a very important player. Now I want to repay that faith on the football field.
"Swansea is a very important football club and will only keep getting better. They were a good side last year and look even better so far this season. I watched the game against West Ham on TV last week and they played some good football. I also liked the atmosphere; the fans are good and together with Michael Laudrup's presence it's an exciting package.''