PSI (Psionics) or PK (Psychokinesis) refers to the psychic powers that appear throughout the Mother series. The numerous PSI-wielding allies and enemies throughout the series each have their own array of PSI powers that they are able to draw from. As characters level up, they will gain access to successively stronger variants of their PSI moves. The power level of a given PSI move is designated by a Greek letter; in ascending order of power, the tiers are Alpha (α), Beta (β), Gamma (γ), Pi (π) or Sigma (Σ), and Omega (Ω). PSI abilities can be compartmentalized into four different categories:

• Assist PSI — Used for creating shields, boosting or weakening the stats of an ally or foe (This doesn't include HP or PP), or inflicting a status ailment.
• Offense PSI — Used for dealing significant, elemental, or even mortal damage to the target.
• Recover PSI — Used to recover the HP or PP of one of the user's allies, as well as curing various status ailments.
• Other PSI — PSI only usable outside of battle. Teleport and Telepathy are examples. (No "Other" PSI moves exist in Mother 3.)

Giygas's species and their allies are known to be proficient at PSI, and George is stated to have been studying the power some decades prior to the events of Mother. In EarthBound, various enemy types—humans, animals, aliens, and even inanimate objects—are able to use PSI, potentially as a result of Giygas's influence. Additionally, the Dalaamese seem to have developed the ability to tap into PSI on their own. In Mother 3, the use of PSI abilities is generally limited to the Magypsies and their students, with rare exception. While the Magypsies have apparently been using it for centuries, it is worth noting that Mother 3 is implied to take place far into the future from the first two games in the series.

The effects of some PSI moves differ from game to game, and their description in a given section will cover what they do in that specific game.

PSI abilities in EarthBound Beginnings

Upon reaching the required level, the member has a 1/4th chance to learn it after every battle regardless of if they win or run away.

• 4th-D Slip — Assist PSI. It is used to escape from a battle with a 100% success rate. However, boss battles can't be escaped from even with 4th-D Slip.
• BrainCyclone — Assist PSI. It inflicts the "confused" status ailment to all opponents. This is the second level of BrainShock, being an early version of Brainshock Ω.
• BrainShock — Assist PSI. It inflicts confused to one opponent. 
• Darkness — Assist PSI. It inflicts the "blind" status ailment to an opponent.
• Def.Down — Assist PSI. It slightly reduces the defense of an enemy. Later levels can target multiple foes.
• DefenseUp — Assist PSI. It increases the defense of a party member by a percentage of their normal Defense. The later level can increase the defense of all party members.
• Healing — Recover PSI. It heals a single status ailment from a party member. Later levels allow this ability to cure different status ailments of a party member.
• Hypnosis — Assist PSI. It inflicts the "sleep" status ailment to an opponent. 
• LifeUp — Recover PSI. It recovers the HP of a party member by an amount which increases with higher levels of the ability.
• OffenseUp — Assist PSI. It increases the offense of a party member by a percentage of their normal Offense.
• PK Fire — Offense PSI. An elemental ability that causes damage to all foes. PK Fire Ω attempts to instantly destroy all enemies.
• PK Freeze — Offense PSI. An elemental ability that causes damage to one foe. Later levels can bring an enemy down to critical HP or target all foes.
• PK Thunder — Offense PSI. An elemental ability that causes damage to one foe. Later levels can target all foes.  
• PowerShield — Assist PSI. It can reflect any offensive PSI or attack at no damage to the user. This targets only one ally.
• PK Beam — Offense PSI that is rumored to be the strongest attack PSI. Deals a significant amount of damage to one or all opponents and may even destroy them instantly. As PK Beam does not have an element attached to it, no Pendant exists that provides resistance against it. However, PK Beam γ can be reflected back at the user by a character carrying a Franklin Badge in their inventory. Mechanical enemies are completely invulnerable to PK Beam γ, regardless of whether it was used on them by the player or reflected back upon them via a Franklin Badge. 
• PSI-Block — Assist PSI. It prevents an opponent from using any PSI. 
• PSI Shield — Assist PSI. It halves all damage. The later level can be used on all allies at once.
• PSI-Magnet — Recover PSI. It steals 10 PP from one opponent.
• QuickUp — Assist PSI. It increases the speed of a party member by a percentage of their normal Speed. 
• Shield-Off — Assist PSI. It removes any shield from one opponent.
• SuprHealing — Recover PSI. It instantly revives a party member to full HP.
• Telepathy — Other PSI. It has various uses on the field, usually to read minds or to talk with animals.
• Teleport — Other PSI. When used, it allows the party to teleport to previously-visited towns, or to Ninten's house. Movement speed gradually increases when Teleport is used, with teleportation occurring after the player successfully avoids obstacles for long enough to reach maximum speed.

PSI abilities in EarthBound

• Brainshock — Assist PSI. It inflicts the "strangeness" status ailment to an opponent. Can target one or more enemies.
• Defense down — Assist PSI. It lowers the defense stat of an enemy to a low or high rate. Can target one or more enemies.
• Healing — Recover PSI. It heals one or more status ailments from an ally. Some status ailments like "Mushroomization" can't be cured. Can be used outside of battle.
• Hypnosis — Assist PSI. It inflicts the "sleepiness" status ailment to an opponent. Can target one or all enemies in battle.
• Lifeup — Recover PSI. It heals a party member to a minimum to maximum amount of HP. Can be used outside of battle, and can target one or more allies.
• Offense up — Assist PSI. It raises the offensive stat of a party member to a low to high rate. Can be used for one or more allies.
• Paralysis — Assist PSI. It inflicts the "numbness" status ailment to an opponent. Can target one or all enemies in battle.
• Shield — Assist PSI. Mitigates or counters most physical attacks. Effective when stacked. Can be used to one or all allies.
• Telepathy — Event PSI. It is used to communicate directly with the mind of another person, and can be used over great distances. Paula uses it to communicate with Ness and Jeff.
• Teleport — Other PSI. When used, it allows the party to teleport to previously-visited towns or settlements. Movement speed gradually increases when Teleport is used, with teleportation occurring after the player successfully avoids obstacles for long enough to reach maximum speed.
• PSI Farewell — Event PSI, used by Poo at the end of the game. It appears to function identically to Teleport β.
• PSI Fire — Offense PSI. An elemental ability that can burn an opponent. Best against ice-based foes. Targets one row of enemies.
• PSI Flash — Offense PSI. Inflicts a random status ailment to an opponent. It can rarely incapacitate a target. Targets all enemies.
• PSI Freeze — Offense PSI. An elemental ability that can freeze to an opponent. Best against fire-based enemies. Targets one.
• PSI Magnet — Recover PSI. It steals a small amount of PP from the target. Can target one or all.
• PSI Rockin — A very strong Offense PSI ability. Its potential damage range is very broad despite having a high cap, and the attack also has a low chance of failing to affect a target at all. Targets all enemies.
• PSI Shield — Assist PSI. Dissipates or counters most PSI attacks. Effective when stacked. Can be used to one or all allies.
• PSI Starstorm — One of the strongest Offense PSI abilities. Deals major damage. Targets all enemies.
• PSI Thunder — Offense PSI. An elemental ability that can inflict numbness, as well as very high damage. Targets a random opponent. Tends to miss often.

PSI abilities in Mother 3

• Brainshock — Assist PSI. It inflicts "strangeness" to one or more opponents.
• Counter — Assist PSI. It reflects 50% of physical damage from an opponent. Some attacks can't be halved, however. Can be used to one or more allies.
• Defense Down — Assist PSI. It lowers the defense stat of one or more opponents at a minimum to a maximum rate. Effective when stacked.
• Defense Up — Assist PSI. It increases the defense stat of one or more allies by a low to high rate. Effective when stacked.
• Healing — Recover PSI. It removes one or more status ailments to one or more allies.
• Hypnosis — Assist PSI. It inflicts "sleepiness" to one or more opponents.
• Lifeup — Recover PSI. It recovers the HP of one or more allies to a range of minimum to a maximum rate.
• Offense Down — Assist PSI. It lowers the attack stat of one or more opponents at a minimum to a maximum rate. Effective when stacked.
• Offense Up — Assist PSI. It increases the attack stat of one or more allies at a minimum to a maximum rate. Effective when stacked.
• Paralysis — Assist PSI. It inflicts "numbness" to one or more opponents.
• PK Fire — Offense PSI. An elemental attack that can target a row of enemies. It rarely inflicts the "burn" status ailment. Effective against Ice-type foes.
• PK Flash — Offense PSI. It can inflict a random status ailment to each of the enemies in battle. It has a low chance of incapacitating a target.
• PK Freeze — Offense PSI. An elemental attack that can target one opponent. It rarely inflicts the "freeze" status ailment. Effective against fire-type foes.
• PK Ground — Offense PSI that targets all enemies in battle. It deals several consecutive attacks based on a percentage of the targets' HP. It has a low chance of "tripping (solidification)" to an opponent.
• PK Love — One of the strongest Offense PSI abilities. Deals significant damage to all opponents.
• PK Starstorm — One of the strongest Offense PSI abilities. Deals major damage to all opponents.
• PK Thunder — Offense PSI. An elemental attack that can randomly attack any opponent. Tends to miss often, and rarely inflicts the "numbness" status ailment. Effective against mechanical-based enemies.
• PSI Magnet — Recover PSI. It steals a low amount of PP from an opponent, to recover the user's PP.
• PSI Counter — Assist PSI. It reflects 50% of PSI damage from an opponent. Some attacks can't be halved, however. Can be used to one or more allies.
• PSI Shield — Assist PSI. It reduces PSI damage from an opponent by 50%. Some attacks can't be halved, however. Can be used to one or more allies.
• Refresh — Recover PSI. It recovers a minimum to a moderate amount of HP to all allies. The effect disappears in a few turns.
• Shield — Assist PSI. It reduces physical damage from an opponent by 50%. Some attacks can't be halved, however. Can be used to one or more allies.

Unused Dummy PSI in Mother 3

These can only be used by the Unused Boss. Double-spaces are present between "PK" and the ability's rank, potentially as a result of formatting codes used when printing the names onscreen, or to accommodate for proper names for the abilities that were never implemented.

• PK  β — Poisons the entire party. Only the unused Dung Beetle and Unworked Clay (the Dung Beetle that drops Silver Dragonflies) enemies have it, and it cannot be learned.
• PK  γ — Similiar to Paralysis, except only the unused Tent Person and Unworked Clay enemies have access to it. It does not cost any PP to use, despite it being possible to steal PP from Tent Person and/or Unworked Clay using PSI Magnet.
• PK  Ω — Only the unused enemies Snow Bunny and Unworked Clay have and can use this. It works like Hypnosis, and cannot be learned.

Trivia

• Oddly enough, EarthBound replaces the "PK" prefix to Offense PSI with "PSI", a prefix ordinarily used only for PSI abilities that interact with PP or other PSI moves in some way. This change does not exist in the official English translation of Mother, which leaves the prefix to Offense PSI moves as "PK".
• In Earthbound, interestingly, the line "(character)'s PP are maxed out!" or anything serving its purpose is not coded to appear. The easiest way to see this is to use a PP restoring item while already having full PP, simply resulting in the text "(character)'s PP was restored by 0."