A belt tensioner comprising a pivoted structure carrying a pulley for rotation about a rotational axis and mounted on a fixed structure for pivotal movement between an increased range of angular movement. The pulley is operatively positioned with respect to its rotational axis and the pivotal axis of the pivoted structure such that the lever arm to the pivotal axis through which the generally constant belt load force acts on the pivoted structure increases at a decreasing rate and then decreases at an increasing rate throughout the range of movement of the pulley between its first and second positions in a direction toward the second position. The spring assembly is operatively associated with the pivoted structure so as to apply a spring torque to the pivoted structure which increases at a decreasing rate and then decreases at an increasing rate throughout the range of movement of the pulley between its first and second positions in a direction toward said second position.