The Spider Guardian is slightly larger than a normal Pillbug, with reddish skin and longer ridges on its carapace. It is able to travel along magnetic rails thanks to the Spider Ball, and it projects a harmful electromagnetic field wherever it goes.
The battle with the Spider Guardian is very difficult and unique since it is fought while Samus is in Morph Ball mode. The Guardian moves back and forth along the magnetic rails, ramming the Morph Ball with its protective energy field. The color of the field changes with the speed of the Guardian, turning green when slowed down, blue at default speed, yellow at high speed and red at maximum speed. Samus must first slow it down by laying Morph Ball Bombs in its path until it slows down, which activates a Bomb Slot nearby. Then, the slot must be triggered before the Guardian wakes up to open a gate to a power conduit on the rails. If done, the Guardian will turn red and rocket across the rails until it hits the conduit and electrocutes itself, taking out a chunk of its health. Once all conduits in an area are hit, the Spider Guardian moves on to the next area and the phase of the battle. Killing the Guardian causes the Darkling to zoom out of the maze and die in a room at the end of the corridors, leaving behind the Spider Ball for Samus to collect.
- Metroid Prime 2: Echoes (First appearance)
- The Spider Guardian is the smallest boss enemy in the Metroid series.
- Because of the crippling amounts of damage it deals, distance from the nearest Save Station, unpredictable movements and being fought with the Morph Ball makes it one of the two hardest bosses in Echoes, the other being the Boost Guardian.
- The Spider Guardian is one of three bosses fought in Morph Ball mode, the other two being the Caretaker Class Drone and Power Bomb Guardian, both of which are also from Echoes.
- The Spider Guardian's shield resembles the Death Ball from multiplayer mode.