Shin Megami Tensei If... is a role-playing video game in which players take the role of the protagonist (an unnamed boy or girl, who appears in later Megami Tensei titles as a girl named Tamaki Uchida). As the protagonist, the player explores both the demon-infested Karukozaka High School and five towers based on the Seven Deadly Sins. During the course of the game, using a wrist-mounted device called a COMP, the player fights demons through a turn-based battle system, using physical and magical attacks to deal damage, as well as healing party members and casting status buffs and ailments on enemies.

Demons can be talked with in battle and recruited if the right conversation is initiated. Once a part of the player's party, they will fight alongside them. In addition to standard demons, the player can ally with one human companion, with three being available on the first playthrough and a fourth unlocking during the second. When the player or their human partner dies in battle, they are granted a Guardian Spirit, who revives them at the last save point and teaches them new skills and alter the player's stats. The Guardian's specialties affect how stats are changed: a higher power ranking will raise the player's strength, but lower magic will decrease their Magic Point meter. The Guardian changes each time the player dies.