A bicycle transmission device basically includes an input rotational shaft, an output part, a transmission mechanism and a switching mechanism. The transmission mechanism includes a first rotating body configured to transmit rotation of the input rotational shaft and a fourth rotating body configured to transmit the rotation of the input rotational shaft. The switching mechanism is configured to selectively switch between a first state in which a ratio of a rotational speed of the output part to a rotational speed of the input rotational shaft is a first ratio and a second state in which the ratio is a second ratio that is different from the first ratio. The switching mechanism includes a one-way clutch and a switching unit.