Detectives are investigating two incidents in Hillingdon which they believe to be linked.
At around 2.45pm on Saturday, 12 September, police received a report of a sexual assault in The Gravel Pits, Northwood.
A woman – aged in her 20s – reported being approached by a male who grabbed her and dragged her into a secluded area of the park.
He then carried out a sexual assault, saying he had a knife on his possession to stop her from shouting out.
A member of the public approached and the victim managed to call for help.
The suspect ran off in the direction of Copse Wood Way and the incident was reported to police.
She is being supported by specially trained officers.
A second incident was reported to police at around 08:15hrs on Sunday, 13 September.
The victim, a woman in her 50s, told officers she had just been attacked in Duck Pond Park, Ruislip.
She said that while walking through the park, a man put his hand over her mouth and said he would stab her if she started to shout.
He managed to pull her to the floor and attempted to drag her off the footpath into a ditch.
The woman screamed and the male ran off in the direction of Pinn Way.
A knife was not seen in either incident.
Detectives believe the two cases to be linked and are appealing for anyone who believes they may have seen anything out of the ordinary to come forward.
The suspect is described as a light-skinned black or Asian man, aged between 20-30 years old, around 5ft 8inches tall, of slim build, with a bald or closely shaven head. He was wearing a dark coloured tracksuit at the time of both incidents.
Detective Sergeant Kevin Pandya, from the West Area Command Unit who is leading the investigation, said: “These two attacks took place in broad daylight and we believe the same suspect is responsible for both incidents.
“We want to reassure the community that our officers are carrying out extensive enquiries to try and identify this man and we are asking for the public’s help to find him.”