The Spaniard began his spell in the Premier League in 2007, leaving Barcelona's famed La Masia academy in order to continue his development under then Liverpool boss Rafael Benitez.
However, having failed to secure regular first-team action during his time at Anfield, the 22-year-old has now elected to return to his homeland.
"I'm proud to announce that I'm joining Alcorcon permanently. I want to say massive thanks to LFC for all they gave me in six years," the attacker wrote on Twitter.
"I will always be a red. It's been my home and I will never forget it. Thanks to all you fans for the support and I hope we celebrate the Premier League this season."
Pacheco made just five appearances in the English top flight for Liverpool, despite impressing regularly for the Merseyside club's reserve outfit.
He will now hope to help Alcorcon seal promotion to La Liga after they suffered defeat in the promotion play-offs last term following a fifth-place finish in the Segunda Division.
Pacheco has been capped by Spain at every level up to Under-21s, but has yet to earn a place in Vicente del Bosque's senior squad.
