The gameplay of Mega Man 8 is similar to that of its predecessors. The player, as Mega Man, must complete various stages that contain side-scrolling action and platforming elements. The player can run, jump, slide, shoot, swim, charge the Mega Buster, and change weapons.  Enemies can be destroyed to reveal items used to refill Mega Man's health and weapon power. Like Mega Man 7, the player first completes an introductory stage and is then presented with four Robot Master stages (Tengu Man, Frost Man, Grenade Man, and Clown Man) to tackle in any order.  At the end of each stage is a boss battle with a Robot Master; defeating the Robot Master earns the player its Master Weapon. Most Robot Masters are weak to either the Mega Buster or one or more Master Weapon, allowing for some strategy in the order the stages are completed. An additional four Robot Masters (Aqua Man, Sword Man, Search Man, and Astro Man) become available once the first four are defeated and an intermission stage is completed.
An addition to gameplay in Mega Man 8 is the ability to display and use multiple weapons onscreen simultaneously. This change introduces a unique way of using weapons strategically. For example, Mega Man can place a Tornado Hold, jump into it and swing the Flame Sword while rising with the air current. The player also has the ability to use the Mega Buster at all times, even when equipped with a Master Weapon. The player can buy new abilities from Roll in Dr. Light's lab in exchange for special bolts found throughout the levels. However, due to a limited number of bolts and powerup slots available, the player must carefully decide which power-ups to buy. Power-ups utilizing Mega Man's dog Rush are won by fighting mid-stage minibosses. Such Rush items include Rush Question, giving the player a random pickup, Rush Charger, which makes pickups rain from the skies, and Rush Bike, used to cross large gaps.