Sigma

Sigma may refer to:
 * Σ, an Ood (DW: Planet of the Ood)
 * Sigma (planet), the home of Dr. Ivan Asimoff (DWM: "The Fabulous Idiot")
 * Theta Sigma, an alias of the Doctor (DW: The Armageddon Factor, The Happiness Patrol, DWM: "Flashback")