Swaps your max HP and MP values.
Description
HP⇔MP materia is a 
Complete Materia in Final Fantasy VII Rebirth that switches the user's HP with their MP.
Obtained[]
HP⇔MP materia is found in the cave with a mako spring on Mt. Nibel in chapter 11, "The Long Shadow of Shinra".
Use[]
HP⇔MP swaps the party member's HP and MP. This allows them to have a much higher MP pool in the thousands, at the cost of a severely limited HP pool in the low hundreds. As such, it is a risky materia to equip. Primarily, the materia will be used when spellcasting frequently, such as when out level grinding. The player can have First Strike Materia or other means of starting battles with ATB, and then immediately defeat groups of enemies with magnified spells (perhaps also linked with 
Swiftcast Materia) without needing to worry about MP running out too quickly. Casting spells on Radiant Ward will make the caster immune to damage.
Having a low HP pool will make it easier to fill limit gauges.
To mitigate the risks, players can use Precision Defense Focus Materia or rely heavily on evasion. Perfect-blocking mitigates damage, but the character gets the limit break bar increase they would from taking the full damage. When using a limit break, the user is immune to damage as well. The player can protect the caster with 
Reraise, which they easily have MP for with HP⇔MP, however, 
Reraise Materia is an endgame materia.
HP⇔MP can be good in hard mode where MP-replenishment is harder. When equipped onto a backliner, the player can have a reliable source of healing between battles.
In most cases the materia will still be too risky to use, but in cases where the user is not in harm's way often, there can be niche uses for it.
Changing materia setups that affect HP or MP when HP⇔MP is equipped may break the effect (likely so the player cannot keep adjusting their MP while using it), and so the player is best off not touching their set-up after setting it up. When the player removes HP⇔MP from a character, their HP will remain low and they need to be healed.
Instead of using HP⇔MP to have MP for endless spellcasting, the player may get a constant spellcasting effect from Synergy Materia, however, the spells cast via synergy will be much weaker.