Officers have released a CCTV image in the hope that someone will recognise the man believed to be involved in both incidents, of women in their 20s.
The first assault happened on Kilburn Park Road, Westminster, near Augustine Primary School at around 10.35pm on Monday June 1.
The victim was followed by the suspect, then he attempted to get into a conversation with her before touching her inappropriately, at least twice, before she ran away into the Queen's Head pub where staff assisted her and the police were called.
The second incident occurred on Saltram Crescent, Westminster, slightly earlier at 9.55pm, on Wednesday June 17.
The victim who was on her way home from work first noticed the suspect as she walked through a park after getting off the bus at Carlton Vale at the junction of Pentland Road.
She continued to walk through an alleyway, believed to be on Malvern Road and turned on to Saltram Crescent, where the suspect tried to talk to her and then touched her inappropriately before pressing himself against her.
The woman pushed the man away and called out for help and he ran off in the direction of the route they had walked.
Once at home the victim contacted police.
The suspect has been described as a black man, in his late teens to early 20s, 5ft 9ins to 6ft tall, of slim build with short black hair, and was wearing a grey hooded top with dark coloured jeans and white trainers.
In the first incident, he was also wearing a black jacket.
The suspect is described as appearing immature and unsure of himself.
Detective Chief Inspector Jane Corrigan, said: "I am very concerned that these events could escalate into a serious sexual assault.
"I would urge anyone who recognises the man in the CCTV image to contact officers as a matter of urgency."
Anyone with information is asked to contact Westminster CID via 101 or Crimestoppers anonymously on 0800 555 111.
No arrests have been made and enquiries continue.