The 32-year-old woman was jogging north on the trail near Spruce Avenue around 6:30 a.m. when she passed a man walking the opposite direction, according to police.
The man grabbed her from behind and thrusted his pelvis against her repeatedly before running away, police said. He was fully clothed during the attack, as was the victim.
The suspect was last seen heading south on the trail toward San Bruno.
He was described as a dark complexioned Hispanic male adult, 20 to 30 years old, around 5 feet 4 inches tall, with broad shoulders and a small waist.
He had a "faux hawk" hairstyle and was wearing black slacks and a snug fitting v-neck t-shirt.
Anyone with information on this suspect or attack should call police at (650) 877-8900.