Montes Secchi (Latin for "Secchi Mountains") is a minor range of lunar mountains located near the northwestern edge of Mare Fecunditatis. This roughly linear formation of low ridges grazes the northwestern outer rim of the crater Secchi, the formation from which this range gained its name (the crater is named after Pietro Secchi, a 19th-century Italianastronomer). The ridges trend from southwest to northeast.
The distinctly triangular Secchi Theta is better known by its original name, "Mount Marilyn"; it was named in 1968 by Apollo 8 astronaut Jim Lovell, after his wife.
The selenographic coordinates of the range midpoint are 3.0° N, 43.0° E, and they lie within a diameter of 50 km. This is smaller than the diameter of the crater Taruntius, located to the northeast of the mountains.