A chain tensioning device is basically provided with a main body, a chain guide and a rotary fluid damper unit. The main body is configured to be mounted to a bicycle. The chain guide is pivotally coupled to the main body to rotate around a rotational axis with respect to the main body. The rotary fluid damper unit is operatively connected between the main body and the chain guide to provide rotational resistance to the chain guide around the rotational axis. The rotary fluid damper unit is a separate unit from the main body, the rotary fluid damper unit being detachably mounted on the main body.