The GAT-X303 Aegis Gundam is a Mobile Suit introduced in the anime Mobile Suit Gundam SEED. It is piloted by Athrun Zala.

Designated as the commander unit of the Alliance's G Project due to its enhanced communication and analysis capabilities, Aegis also has heavy attack power as well as Phase Shift armor and can transform into a mobile armor (MA) due to its X300-series frame. In MA mode, it has two different sub modes, 'Cruise' and 'Attack'. When in 'MA Cruise Mode', Aegis has higher speed and acceleration, while in 'MA Attack Mode', it can destroy enemy ship/mobile suit with its "Scylla" 580mm energy cannon directly or grab onto them with its four limbs first then destroy them using the cannon. Other weaponry includes two head-mounted CIWS, a hand-held beam rifle, a shield and a beam saber in each limb.

Aegis also has a large sensor unit at the top of its head and unlike most suits, its main thrusters seem to be located in the side binders on its waist. Due to the transformation system, Aegis's internal structure is more complex and it is for this reason that there are no mass production variants of the suit. The three different modes (MS Mode, MA Cruise Mode and MA Attack Mode) also means that Aegis's pilot needs to have higher skills to be able to use the suit effectively.