Killer Move: Serious Series

Killer Move: Serious Series (マジシリーズ, Hissatsu Majishirīzu) is a fighting style used by Saitama which consists of a series of his "serious" moves.

Overview
Whenever Saitama wants to put more effort into fighting opponents, he uses these moves, dubbed "serious". He usually puts on a serious look on his face while performing these moves. Overall, the clear difference between his "serious" and "normal" attacks is the massive increase in power and destructive capability. Like with the case of his "normal" punches, he can unleash his Serious Punches that vary in potency due to him having great control over his power. Saitama himself does not actually consider these moves to be anything special. This fighting style is improvised and usually made up on the spot while in battle. Its simplicity seems to reflect his lack of interest in learning martial arts techniques.

After becoming influenced by "God", Garou is able to duplicate the power of Saitama's Serious Punch by using Mode: Saitama. Doing this changes his face into that of Saitama and allows him to throw an equally strong Serious Punch of his own. This makes him the second user of this unique fighting style, although Garou has only been shown using Serious Punch.

Known Moves

 * Serious Punch (マジり, Maji Naguri): The first serious move used by Saitama in the series. He simply throws an extremely powerful punch. One of his serious punches was so powerful that not only did it completely negate Boros's Collapsing Star Roaring Cannon, a beam capable of wiping out an entire planet, but the shockwave from the punch alone was also powerful enough to part the clouds on a global scale, as well as deliver the final fatal blow to Boros. The dying Boros also declared that Saitama's fatal attack on him was not even anywhere near Saitama's true power and that Saitama was still holding back. Saitama also used this move to completely annihilate and disintegrate Elder Centipede, a Dragon-level threat and one of the biggest monsters in the series. When used against Evil Natural Water, who has become one with the ocean, he was able to dissipate its form and split the ocean entirely. A direct collision between two Serious Punches, one from Garou who gained Saitama's strength level thanks to his copy ability and the other from Saitama himself, could produce a Serious Punch² which outputted so much destructive energy that it threatened to destroy the Earth. Furthermore, after being diverted by Blast and three of his companions, the energy of Serious Punch² could travel into outer space where it (presumably) resulted in the destruction of numerous star systems in the far distance, with the recoil launching both Saitama and Garou to Jupiter's moon Io.
 * Omnidirectional Serious Punch (マジり, Zenhōi Maji Naguri): Saitama leaps from multiple angles to create a series of afterimages which all then surround and strike the opponents with a Serious Punch, seemingly leaving no room for them to escape. He first used this attack against Cosmic Fear Mode Garou, who attempted to flee through a hyperspace portal to avoid the attack.
 * Serious Side Hops (マジび, Maji Hanpukuyokotobi; Viz: Serious Sideways Jumps): Saitama rapidly hops from side to side, creating a wall of afterimages. He is able to walk forward while doing so, and the shockwaves caused by it were enough to easily render unconscious Speed-o'-Sound Sonic, even with his 10 afterimages.
 * Serious Squirt Gun (マジ, Maji Mizudeppō; Viz: Serious Water Gun): Saitama uses his hands in liquid to squirt a super high-pressured stream. He used this move while submerged in lava to completely overpower Orochi's Gaia Cannon which was an extremely powerful attack that drew energy from the very core of the Earth itself.
 * Serious Table Flip (マジちゃぶし, Maji Chabudai-Gaeshi): Saitama digs his fingers into the ground and flips a large amount of ground into the air. He first used this technique against Garou while they were on Io, which completely ripped apart the moon, launching its fragments into space. In the original webcomic, this move was first used on Garou in Z-City, which flipped over the Monster Association Headquarters. The effect was so great that Garou's sense of direction was distorted.
 * Serious Sneeze (マジ, Maji Kushami): Saitama simply lets out a sneeze. In his fight with Garou on Io, one of Jupiter's moons, Saitama's strength increased enough that a mere sneeze from him ended up devastating the large planet Jupiter when struck.
 * Serious Headbutt (マジ頭突き, Maji Zuzuki): Saitama headbutts his target. In the original webcomic, he uses this move to counter a punch from Garou and break his arm.

Trivia

 * In the original webcomic, there were only Serious Punch, Serious Side Hops, Serious Table Flip and Serious Headbutt.
 * Murata once joked that if Saitama missed the moon in his fight with Boros, Saitama could use a serious fart to get back to earth.