The third season of the Canadian science fiction–fantasy television series Sanctuary, premiered on Syfy in the United States on October 15, 2010 and consists of 20 episodes. Created by Damian Kindler, the series was adapted from a series of webisodes released in 2007. The increased number of episodes in this season allows the producers to include numerous story arcs. The second half of the third season premiered on April 15, 2011 until it was moved to Monday nights on April 25, 2011.

The Sanctuary team deal with the aftermath of the gigantic tidal wave released by Big Bertha while Will's life remains in balance but soon the errand comes to an end once Will leads one final plea to "Kali" A.K.A. Big Bertha in his near death experience to stop the destruction. Following Will's return to life two abnormals appear to him to rekindle his memory and in doing so Will remembers an important message that Gregory sent for Helen to find an underground city with great technology and secrets.

Later Nikola sends an SOS call and after his rescue continues to help Helen in discovering the secrets of the city which eventually leads them to a Hollow Earth atlas. Soon Adam Worth, a former acquaintance, tricks Helen and causes radiation poisoning (that should kill both Helen and Adam in a few weeks) so that she may show Adam the Hollow Earth. Upon Adam's interference Helen, Will, Henry and Kate enter the city to save Gregory and find a cure for the poisoning but only to be executed immediately when caught while Adam imprisons John, leaving Tesla and Big-Guy alone in the mysteries. Magnus is then revived by Ranna for answering and Magnus finds out that she needs her to help an abnormal from the events of 'KALI', thereby Magnus and the gang is revived and she given the cure and is reunited with her father, together Gregory and Helen save the abnormal and reach the surface back again while John kills Adam.

The show returned to its original format, featuring a new short story for every episode until Tesla and Magnus discover a Praxian Stronghold. This is revealed to have been taken by Vampires and as such was a Vampire Stronghold where a whole army of Vampires and their queen was buried to be awakened someday. Magnus views this as a threat and destroys the stronghold, but not before restoring Tesla to a Vampire again.