"Attack on Cloudbase" is the 31st episode of Captain Scarlet and the Mysterons, a British Supermarionation television series created by Gerry and Sylvia Anderson and filmed by their production company Century 21 Productions for ITC Entertainment. The series' penultimate episode, it was written by Tony Barwick and first broadcast on 5 May 1968 on ATV London.

Set in 2068, the series depicts a "war of nerves" between Earth and the Mysterons: a hostile race of Martians with the ability to create functioning copies of destroyed people or objects and use these reconstructions to carry out specific acts of aggression against humanity. Earth is defended by a military organisation called Spectrum, whose top agent, Captain Scarlet, was murdered by the Mysterons and replaced with a reconstruction that later broke free of their control. The double of Scarlet has powers of self-repair that enable him to recover from injuries that would be fatal to any other person, which make him Spectrum's greatest asset in its fight against the Mysterons.

"Attack on Cloudbase" begins with Spectrum fighter pilot Symphony Angel being forced to eject into the Sahara when her aircraft is damaged. Later, a fleet of Mysteron spacecraft enter Earth's atmosphere and launch a devastating assault on Spectrum's airborne headquarters, Cloudbase. Filmed in late 1967, the episode underwent a number of script changes prior to shooting. Gerry Anderson chose Ken Turner to direct due to its unusual nature of the story. The episode presented Century 21's special effects team with several technical challenges, including a scene in which a miniature Angel aircraft overflies the puppet of Symphony Angel in the same shot. For the attack sequence, only a small number of spacecraft models were made; flashing light bulbs were placed in the backdrop as a cost-effective way to create the illusion of a larger fleet. Series composer Barry Gray wrote a full score after Anderson decided that none of the incidental music in Century 21's library suited the episode's grim tone.

Critical response to "Attack on Cloudbase" has been mostly positive, with one commentator naming it the best episode of Captain Scarlet. Some criticism has been directed at its ending plot twist. Anderson, who likened the episode to a black comedy, praised Barwick for his "humanised" writing of the puppet characters. In 1980, the New York office of ITC re-edited the episode to form the ending part of the Captain Scarlet compilation film Captain Scarlet vs. the Mysterons.