A mobile app is one product governed by four separate legal regimes at once, and each protects a different slice of what you built. Trademark secures the name and icon that users recognize; copyright covers the code and creative assets; a patent may reach genuinely novel functionality; and trade secret guards the logic you never ship. This guide maps every regime onto the part of an app it actually reaches, sequences the filings by cost and deadline, and marks the traps that quietly transfer ownership away from the founder who wrote the check. It is current through early 2025, reflecting Google v. Oracle on interface copyright, the post-Alice eligibility guideposts, the 2025 USPTO fee restructuring, the Defend Trade Secrets Act, the post-Epic app-store settlement, and the Copyright Office's refusal to register machine-authored material. Worked hypotheticals and a lifecycle checklist connect the doctrine to the decisions a developer makes before shipping.