Abdennour had been linked with a move to Barcelona earlier in the summer but has now joined los Che for an estimated 25 million euros ($27.6 million), plus €5 million ($5.6 million) in add-ons.
The Tunisia international, 26, passed a medical on Saturday and signed a contract until 2020 at Mestalla, with Valencia having coincidentally qualified for the Champions League group stage at the expense of Monaco on Tuesday.
"Valencia are a great club in Spain and in Europe, I had no doubts," Abdennour told his new side's official website. "I am happy to be here in a serious and ambitious project, I want to thank the technical staff, president Lay Hoon and owner Peter Lim.
"I am going to give anything to have a good season with my new teammates and I hope to win a lot of games.
He added: "I know the club and the players well, everybody knows Valencia because they have a lot of trophies and are a great football club. I'm very happy to come to the best league in the world and I hope to do well with the team."
Lay Hoon said: "Aymen is one of the best defenders in Europe; strong, young and with great potential. I am happy because he has shown his interest in and committment to our project from the first moment."
Abdennour started his career with Etoile du Sahel and moved to Europe with a loan spell at Werder Bremen in 2010, before joining Toulouse a year later and moving to Monaco - initially on loan - in 2014.