Police said the attacks took place around the North Street area of the city centre last week.
A woman was sexually assaulted after leaving a nightclub in the early hours of 13 July.
Another woman was attacked as she walked along a riverside path near Bridge Street on 15 July.
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Det Insp Mark Pearson from North Yorkshire Police said both assaults happened in the early hours of the morning.
"At the times these incidents happened, and in that part of the city, there's a good chance bar and door staff were clocking off or having a break, or revellers were making their way home after a night out," he said.
"I'd appeal to these people in particular to carefully jog their memories of the dates in question."
A 32-year-old man has been arrested in connection with two sexual assaults in York.