Arks Mechanicus are a class of incredibly large, nigh-mythical Void ships of the Adeptus Mechanicus Fleet. These ships are said to endlessly search the stars as part of the Adeptus Mechanicus' Quest for Knowledge.
Filled with the best technology that the Adeptus Mechanicus can create, the weapons of an Ark Mechanicus are stronger and longer-ranged than even Imperial Battleships, and their defences are significantly stronger. An Ark Mechanicus class vessel is equipped with a Nova Cannon, four Lance batteries, and two Weapons Batteries. This makes these rarely-seen ships truly mighty opponents on the rare occasions that their journeys are interrupted by battle. So valuable is an Ark Mechanicus that the destruction of an entire Forge World, just to reawaken the ship's Machine Spirit is considered a small price to pay.[2b] The extreme rarity of these venerable ships is such that to discover one intact is a miracle to rival finding a fully functioning STC system. as only a handful of Ark Mechanicus are said to exist.[2b]
Atleast one Ark Mechanicus, the Speranza, secretly contains a fully functional and still updating STC. Archmagos Lexell Kotov theorized that all Ark Mechanicus secretly contained intact STCs and STC databases, the very things they were sent to find, but the Mechanicus just do not realize it and do not know how to access those functions. The knowledge of the STC was wiped from Kotov's mind upon disconnecting.
Ark Mechanicus are among the largest ships of any in the Imperium. The extreme mass of these vessels create a distorted gravity field equivalent to that of an unstable moon. Wider than a Dominator-class Cruiser is long, an Ark Mechanicus contains a hive’s worth of manufactories, refineries, power plants, laboratories, testing ranges, chemical vats, and gene-bays. The engines of these vessels are larger than the full mass of most starships.
- Archimedes Rex 
- Ark Majesty - Flagship of Fabricator General Kubik.
- Caestus Metalican - Flagship of Magos Dominus Faustinius
- Encoronada - From the Forge World Satzica Secundus. Status: Controlled by the Dark Mechanicum
- Iron Revenant - Former flagship of Archmagos Dominus Belisarius Cawl.[8a] Status: Destroyed.[8b]
- Omnissiah's Victory - Took part in the Noloptis Crusade (M33).
- Opus Machina - Flagship of Fabricator Locum Engra Myrmidex of Satzica Secundus.[9a] Status: Destroyed[9b]
- Patternmaster - From Segmentum Pacificus Batllegroup 9876 Redeployment Fleet
- Pent-Ark - Commanded by Clain Pent of the Dark Mechanicus.
- Red Ark - Assigned to Baal. Trapped in the Olethros Space Hulk. Status: Destroyed
- Speranza - Flagship of Magos Lexell Kotov[2a]
- Zar-Quaesitor - Flagship of Archmagos Dominus Belisarius Cawl
- 1: Battlefleet Gothic Resources: Ships of Mars pg. 7 http://www.bolterandchainsword.com/argos/BFG/amfleet.pdf
- 2: Priests of Mars (Novel),[Needs Citation]
- 3: The Beheading (Novel) Chapter 5
- 4: Baneblade (Novel), Epilogue
- 5: Shadowsword (Novel), Chapter Three
- 6: The Lost and the Damned (Novel) Chapter 17
- 7: Belisarius Cawl: The Great Work (Novel), Chapter 5
- 8: Gathering Storm: Fall of Cadia
- 9: Adeptus Mechanicus: Skitarius (Novel)
- 10: Adeptus Mechanicus: Tech-Priest (Novel), 1000, DENTRICA I OF I, +NECESSITY BEGETS INVENTION+
- 11: Warhammer 40,000: Mechanicus
- 12: Space Hulk: Deathwing (Video Game)
- 13: Nocturne (Novel), Chapter Two, Part I