Galactus possesses the immeasurable Power Cosmic and is one of the most powerful beings in the Marvel Universe. He has even been referred to as "the most powerful creature in the universe." As such, Galactus is able to employ the limitless cosmic energy within him to produce virtually any effect he desires. A few of the abilities he has displayed are:
Godlike Strength: Galactus possesses an astronomical amount of super strength.
Godlike Stamina: Galactus can partake in physical streneous action, like battles, for an undefined period of time.
Godlike Speed: Despite his size, Galactus can move at speeds that greatly surpass the speed of light.
Invulnerability: Galactus is immune to all diseases, ailments, and toxins, and, he is nearly impenetrable towards almost every force (however he became a zombie in Marvel Zombies).
Levitation: Galactus can traverse or glide through the air using sheer will.
Size-Alteration: Galactus can manipulate his size, density, mass and ratio at will, allowing him to reshape his body to any physique type he wishes.
Molecular Restructuring: Galactus can manipulate, reshape and rearrange the molecular complexions of any organism, allowing him to rebuild or redesign things internally.
Matter Transmutation: Being a cosmic being, Galactus can control matter in any object which allows him to turn something tangible like lead, into something intangible like water.
Cosmic Teleportation: Galactus can make himself, other people, and obejcts appear anywhere in the entire universe that he wishes with a mere thought.
Cosmic Force-Fields: Galactus can erect near-impenetrable energy shields.
Cosmic Energy Projection: Galactus can project extremely powerful energy emissions with incalculable force.
Interdimensional & Intradimensional Portals: He can create wormholes between dimensions and within dimensions.
Cosmic Telepathy: Ability to read or communicate with other minds across universal distances.
Cosmic Telekinesis: Ability to move virtually unlimited amounts of mass through mental concentration.
Cosmic Awareness: Galactus can sense disturbances or changes within the cosmic ether on a universal scale.
Resurrection: He can raise the dead. This ability also allows him to raise himself from death.
Creation: Galaxtus can create complex, sentient, biological life-forms from nothing.
Soul Manipulation: Galactus can manipulate the souls of living beings.
Recreation: Being a cosmic being, Galactus can create an entire Solar System, including populations, the planets, etc. in every detail.
Power Bestowal: Galactus' heralds, whom he has endowed with a minuscule fraction of his cosmic power, are able to manipulate matter and energy in ways that are far beyond human comprehension.
Vitakinesis: Galactus is able to heal himself and others from all physical wounds although this may simply be molecular reconstruction.
Galactus, as the oldest known living entity in the universe, arguably possesses the universe's most advanced intellect. Reed Richards has admitted an inability to fully comprehend even the most rudimentary devices found within Galactus' Worldship Taa II. The Worldship itself is a solar system-sized construct that is said to surpass any natural world in beauty and splendor and yet within it's computers is contained knowledge so unfathomable and far reaching it can grant awesome power to those who come into contact with it. (see Korvac)
Incalculable, when sated Galactus possesses a level of strength beyond the human ability to measure of quantify. Additionally, Galactus can use the Power Cosmic to further enhance his strength to truly infinite levels.
Cosmic Hunger: Galactus requires unknown energies obtainable only from a certain type of planets in order to sustain his life. The suitable type of planet need not to be one where life is present, but it must have the potential to support life. When recently sated Galactus is effectively omnipotent; however, the longer he goes without feeding the more his strength, power, and cosmic awareness wane. Indeed, over one month after consuming a suitable planet his power will have already been reduced exponentially, although in such a hungered state his powers are still far beyond mortal comprehension. In recent times Galactus has been known to go without feeding for numerous months at a time bringing him to the brink of death and reducing his power to such a degree he actually becomes partially susceptible to direct attacks from superhuman teams such as the Shi'ar Imperial Guard, Avengers, the Fantastic Four etc although even in this significantly weakened state of power, he is still more powerful than the superhuman teams combined and still one of the most powerful beings in the Universe. When the universe was still young, Galactus could go without feeding for over century before his hunger weakened him to a significant degree, why the intervals between his feedings has decreased over the millenia is unknown.
Galactus often employs his Elemental Converter when devouring planets to help him in the conversion of matter into energy. Although he does not require this device to devour planets and will forego its use in certain scenarios, it makes the conversion of energies more efficient. Galactus also wears a full-body suit of armor constructed from an unknown material of his own design that helps him to regulate and control his personal energy. If he were to remove the armor for an extended period, his vast cosmic energies could run rampant and potentially give him the appearance of a star.
Although Galactus is capable of space travel via his own power, he disdains the needless expenditure of power and prefers to travel in starships of his own design. Though Galactus has many different starships in his personal fleet his preferred long range vessel is his sphere-shaped starship (over a mile in diameter) which is capable of trans-light speeds and interdimensional travel. Galactus has even on rare occasions been shown to travel in his solar system-sized Worldship, Taa II.
Galactus often employs a robot called the Punisher—which possesses vast superhuman strength, speed, and durability—to battle adversaries whose power levels do not warrant his direct attention. Galactus also keeps in his possession a weapon called the Ultimate Nullifier. This weapon has the ability to destroy any target the wielder can mentally envision. In the hands of Galactus, due to his nearly omniscient and cosmically aware intellect, the Ultimate Nullifier is capable of destroying entire universes and even entire timelines from beginning to end. It was revealed in the Abraxas story arc that the Ultimate Nullifier is actually an aspect of Galactus himself.