Galaxy 4 is the first serial of the third season of the British science fiction television series Doctor Who. Written by William Emms and directed by Derek Martinus, the serial was broadcast on BBC1 in four weekly parts from 11 September to 2 October 1965. In the serial, the First Doctor (William Hartnell) and his travelling companions Vicki (Maureen O'Brien) and Steven (Peter Purves) arrive on an arid planet, where they encounter the beautiful but dangerous Drahvins and the hideous but friendly Rills, two crash-landed species in conflict with one another. Both species wish to escape as the planet is set to explode in two dawns, but the Drahvin leader Maaga (Stephanie Bidmead) only wants her people to make it out alive.

Emms, an avid Doctor Who viewer since its beginning in 1963, was commissioned to write Galaxy 4 by outgoing story editor Dennis Spooner after submitting an unsolicited story idea. His idea was to show two conflicting races—one beautiful and one ugly—and flip convention by making the beautiful race evil. Spooner's successor Donald Tosh handled the bulk of the rewrites. Although Verity Lambert was credited as producer, Galaxy 4 was the first serial to be produced by her successor John Wiles. Mervyn Pinfield was originally assigned to direct the story, but his failing health shortly into production prevented him from continuing, and he was replaced by Derek Martinus, a new director. Galaxy 4 was the penultimate serial of the show's second recording block, but was pushed to open the third season. Filming took place at the Television Centre in July 1965.

Galaxy 4 received high viewership numbers, with an average of 9.9 million viewers across the four episodes; and the third episode became the most-viewed of the third season with 11 million viewers. Contemporary and retrospective reviews were generally positive, with praise for its concept and originality. The videotapes of the serial were wiped by the BBC in the late 1960s; the third episode was recovered in 2011, but the other three episodes remain missing. Galaxy 4 received print and audiobook adaptations, and was released on VHS and DVD with reconstructions of the missing episodes using telesnaps and off-air recordings; an animated version of the serial has also been released on DVD and Blu-ray.