|Founded||2030-2031 (exact date unknown)|
|Headquarters||Amadeus base, location unknown (somewhere in Oceania)|
|Area of Operations||Worldwide|
|Enemies With|| Regular Army|
Ptolemaic Army (Metal Slug Attack)
Rebel Army (Metal Slug Attack)
|Affiliated With||Rebellion Army (Metal Slug 4)|
The Amadeus Syndicate serve as the main antagonists of Metal Slug 4. With the money they were able to obtain from their banks around the world, they became a very rich company. Their leader, Amadeus, was a friend of Morden, and when the Rebel Army lost their funding (after the events of Metal Slug 6) the Syndicate helped them by supplying the Rebel Army with more troops, weapons, and vehicles.
The corporation was destroyed during at the end of the White Baby Crisis, where the Regular Army infiltrated their headquarters and defeated Amadeus, causing the base to self-destruct.
Amadeus Infantry: Have the same base sprite as ordinary Rebel Soldiers, but there are also soldiers in blue suits, seen below. Because they rappel down from the ceilings and are generally better equipped than their green counterparts, they may be some kind of Special Forces unit, or honour guards.
AR-10 MG Infantry: Armed with 7,62mm AR-10 Autorifle, they are the elite legion of the Amadeus Syndicate. Some of them have the honour to serve as personal guards of Amadeus. They are very dangerous in comparison to other infantry.
Amadeus Corporals: Much like the MG Infantry, Amadeus Corporals use AR-10s to battle the enemy, though they are also capable of crouching for better firing and accuracy. They wear a dark magenta version of the standard blue suit and only appear in Metal Slug Attack.
Amadeus Soldier (Bazooka): Soldiers on board of the Brave Guerrier. Is still unknown if their skills surpasses the Rebel and Ptolemaic bazookas, but due to the special attack charge of Amadeus they can surpasses in numbers, more if are with spawners on the same deck.
- With the introduction of the Infantry as playable units in Metal Slug Attack, their uniform colors were changed from blue to violet. This is likely to avoid confusion with the Regular Soldier's blue uniforms.
- The Amadeus Syndicate appears in Metal Slug Attack as a part of an Independent Faction.