Radical Fishing is a fast-paced fishing game that consists of two parts. First the player lowers the hook as low as possible (up to 500 meters below sea level) and then tries to pull in as much fish as possible Katarmari-style when the line is reeled in. During that phase the hook can still be moved left to right to catch the fish. Once the surface is reached, all fish are launched into the air and need to be shot down to earn cash. Cash is spent at a shop to acquire all sorts of upgrades such as a lamp to see in the dark, a better line to fish deeper, extra boot, or stronger guns to take down the fish more quickly.

Each layer in the sea introduces new types of fish. A newly-discovered type is stored in the fishopedia. It takes however, a number of upgrades to reach the deeper parts of the ocean; not everything is available right away. A few more mechanics add to the gameplay. While lowering the hook, all fish need to be avoided. As soon as a fish touches it, the line is reeled in immediately. Later parts of the game introduce upgrades that provide second chances or the ability to start at a certain depth right away. Certain fish are also aware of the hook and will attempt to chase it down. While lowering the hook, the player has access to a boost meter to gain depth more quickly. This is not without risk as it increases the chance of bumping into a fish. Finally, not all fish launched into the air can be converted into cash. If they drop back into the sea without being shot, the money is lost. The same gameplay is repeated until everything in the game is unlocked.