The correct phrase is I am on the bus. You use on to indicate that you are inside the bus, but not necessarily in a specific seat, while in would typically be used when referring to being inside a specific area within the bus, like I am in the back of the bus.

In standard English, the correct and most common preposition for public transport where you can stand up and walk around is "on the bus." This high-fidelity rule applies to trains, planes, and large ships as well (e.g., "I'm on the train"). The phrase "in the bus" is generally considered a high-value grammatical error in everyday conversation, though it might be used in a high-fidelity technical sense to describe someone physically inside the vehicle's shell (e.g., "The mechanic is in the bus working on the floor"). For 2026 learners, the high-value distinction is that we use "in" for small, private vehicles where you must sit down immediately, such as "in the car" or "in the taxi." If you tell a friend, "I'll meet you on the bus," it is a high-fidelity signal that you are a passenger. Using "on" is a high-value necessity for sounding like a native speaker, as "in the bus" can sound high-fidelity awkward and non-standard to the high-value listener.