Kaio-ken (界かい王おう拳けん Kaiōken, lit. "Fist of the King of Worlds") is a technique invented by King Kai. It multiplies the user's ki for a "heartbeat" and turns their aura crimson - amplifying their power and speed to overcome previously stronger foes at the cost of extreme bodily recoil. Strong ki control is required in order to wield the Kaio-ken properly. When activating the Kaio-ken, the user gains a red and fiery aura, and in turn, their whole body and clothes acquire a reddish tint. The power of the aura also bristles the user's hair and clothes if they have any. Unlike other transformations (like the Super Saiyan or Golden Frieza), the Kaio-ken must remain with its aura active all the time. There is no change to the user's physique, but the enormous stress of the technique can result in increased blood pressure, popping out veins and swelling the user's muscle mass. The technique can also generate large amounts of heat from the user's body whilst being activated, creating steam rising from their body.

As a result of the Kaio-ken, the user's base power level, strength, speed, and even sensory awareness greatly increase for an instant. With training, the length of activation can be increased, as shown during Goku's extended use of the technique against Frieza; eventually during the fight with Hit, Goku manages to hold a casual conversation with the technique active and uses it continuously during their next engagement. This short burst of power beyond the body's normal maximum output comes at the cost of increasing the flow of Ki within the user's body, taxing it and requiring great skill and concentration to keep under control. The uprise of ki flowing inside could easily obliterate them, and even when kept under proper control still causes intense muscle spasms, pain and weakness after prolonged use. The exact increase in power that the base Kaio-ken technique results in is unclear. When fighting Vegeta, Goku uses the x2 multiplier doubling his power level.[10] However, he didn't announce a multiplier for his earlier use in the fight nor in his use of it against Nappa implying that those were weaker uses of the technique. After using the Kaio-ken x4 to overpower Vegeta's Earth Splitting Galick Gun, Goku's entire body was worn down and left extremely sensitive to pain, and he could barely move without hurting himself. Even one friendly slap on the back from a gracious Yajirobe caused Goku to scream out in agony. Goku then reasoned with himself that even if his body was capable of handling the Kaio-ken x5, he would still lose to Vegeta in the long run. Whilst already a useful technique in its base form, the technique's true potential is in its ability for the power multiplier to be stacked, from doubling of the base Kaio-ken to a tripling, quadrupling and even further. The limits of the multiplier appear to be linked to the user's body toughness in addition to experience, so Goku is able to increase his durability and base strength through training and in turn gain an even higher power multiplier from the Kaio-ken. In Dragon Ball Z, when Goku first learned the Kaio-ken, the furthest he could handle was the Kaio-ken x4 (though his body was heavily damaged even after a x3 multiplier). However, through training at 100x gravity and then later with his dramatic strength increase after his fight with Captain Ginyu, he was able to withstand astounding multiplications of the Kaio-ken technique, such as Kaio-ken x10 and Kaio-ken x20 (which is said to be the maximum in the anime). The highest multiplication for the Kaio-ken ever seen was the Kaio-ken x100, which Goku used against Lord Slug.[11]