The RX-178 Gundam Mk-II is a mobile suit introduced in the anime series Mobile Suit Zeta Gundam. It is a successor of the famous RX-78-2 Gundam that served in the One Year War.

A direct descendant of the RX-78-2 Gundam developed by the Titans, the Gundam Mk-II is the first mobile suit to utilize the new movable frame concept. In this new concept, rather than merely serving as a structural framework, the movable frame incorporates all the mobile suit's vital mechanisms, including the actuators and control systems required for movement, and excludes the mobile suit's weapons, armor, and propellant tanks. This approach dramatically improves the mobile suit's responsiveness, reliability, and energy efficiency, as well as making it far more agile than traditional designs.

Another characteristic of the Gundam Mk-II is that some of its legs' movable parts are exposed and this design feature is based on the prevailing idea at that time that avoiding being hit by the enemy is the preferable choice. The origin for this idea is because no existing armor can fully protect against beam weaponry, thus it is better to increase the mobile suit's mobility by weight reduction rather than equip more armor which would lead to a higher weight and lower mobility.

The mobile suit also has what had become the standard 360-degree panoramic screen and linear seat fitted in a circular cockpit that also functions as an escape pod. The multiple panels of the panoramic screen make up a single 360-degree display, covering the entire inner surface of the cockpit. The image displayed on this screen is a computer simulation constructed from the cameras distributed around the mobile suit's exterior. The viewpoint is that of the mobile suit's head, and the image is carefully edited to remove the mobile suit itself. The linear seat is suspended in the center of the cockpit, supported by a linear arm which electromagnetically counters G-forces and impact shocks in order to protect the pilot. Together, the panoramic screen and the linear seat gives the pilot a larger field of view and eliminates blind spots.

The most notable flaw of the Gundam Mk-II is that it is armored using the obsolete titanium alloy ceramic composite. Another flaw is that the movable frame's leg sections have rigidity issues associated with the strength of the structural material used, and this problem persisted even after six design revisions. Despite this, for a brief time period, the Gundam Mk-II's movable frame makes it one of the most agile mobile suits available. Three Gundam Mk-II were produced in total. They were later stolen by the Anti-Earth Union Group (AEUG) and given a new paint scheme. Despite being used by two different factions, the Gundam Mk-II's weaponry remains mostly unchanged. Its only fixed weapons are two beam sabers, but it can carry a vulcan pod, a shield, a beam rifle, a hyper bazooka and a clay bazooka if necessary.