In one of the attacks, a 27-year-old woman spotted the suspect looking up her skirt at the Lexington Avenue/59th Street station at around 1:45 p.m. Saturday.
The man was on a knee on the platform when she saw him. She screamed in Spanish, and he fled.
In the other attack, police said the man attempted to rape a woman in an apartment building vestibule in East Harlem.
Police Commissioner Ray Kelly said detectives suspect the "small" suspect, described as between 4-foot-11 and 5-foot-3 inches tall may be a restaurant worker.
He tends to strike on off hours and may not have much interaction with the general public.
All of his victims were described as women in their 20s and early 30s.
Police are circulating a photo of the suspect, caught on a subway surveillance camera inside the 77th Street station on the Lexington Avenue line.