A rudder assembly for a boat has a securing base, a rudder board and at least one engaging bolt. The rudder board is mounted detachably on the securing base and has a bottom and two rudder hooks. The rudder hooks are formed respectively on two ends of the bottom and are connected securely to the securing base with the at least one engaging bolt. The at least one engaging bolt is mounted detachably on the securing base and engages respectively at least one of the rudder hooks on the rudder board. Accordingly, a rudder board can be conveniently detached from the securing base even while an inflating boat is at an inflated condition.