Chapter 167 (Online): I.o.

A breath before the two Serious Punches land, Blast opens a portal and drags both combatants into space. Once their fists meet outside the atmosphere, the released energy threatens the world below, but Blast and his allies from other dimensions manage to redirect the force and spare the planet. The backlash then hurls Saitama and Garou across the void.

They come down on Io, a moon of Jupiter, and the fight resumes at once. Saitama presses the attack while Garou leans on his portals, only for Saitama to outmaneuver every trick and demand he fight with his fists.

Blast fights to hold the portal steady against the shockwave, and his allies marvel that anyone besides him can withstand Garou's divine power. The two rivals crash onto Io, where Saitama, thinking he can cut loose, decides to keep Genos' core in his suit as a safeguard. He brushes off Garou's suggestion to find a way home, insisting that beating him comes first.

When Garou opens a portal beneath Saitama, the hero kicks it aside and knocks him off the moon. Garou returns through one portal and spawns another at Saitama's back, but Saitama grabs the portal's rim, flips it, and punches him down again. Garou then rings Saitama with portals and rains a flurry of martial arts strikes, closing with Nuclear Fission Gravity Knuckle that blows Io apart.

Untouched despite his ruined suit, Saitama answers with Serious Table Flip, tearing up the lunar crust and tossing Garou helplessly among the debris. His Omnidirectional Serious Punch hammers Garou from every angle, and Saitama even follows him through a portal to keep hitting. Finally, Garou activates Mode: Saitama, and the two land simultaneous punches to each other's face.

Released online on July 7, 2022, this chapter uses the online title I.o., which Viz Media prints as Io, a nod to the Jovian moon where the battle relocates. It belongs to the Monster Association Arc and adapts parts of webcomic chapters 89 and 90. Saitama and Garou appear together on the cover.