Marilla Brook is a long third-order tributary to West Branch Tunungwant Creek. This is the only stream of this name in the United States.
According to the Geographic Names Information System, it has also been known historically as:
Marilla Brook rises about southwest of Bradford, Pennsylvania, and then flows generally east to meet West Branch Tunungwant Creek about 1-mile southwest of Bradford, Pennsylvania.
Marilla Brook drains of area, receives about of precipitation, and is about 86.80% forested.