Shortly after 3.15pm on Saturday, January 9, a man travelling on the number 16 bus asked a female passenger without a face mask to put one on.
The woman began shouting as she approached the man before spitting towards him from a foot away.
She was then told to leave the bus by the driver and got off in Booth Lane South.
Officers have now released CCTV images of a woman they believe may have information relevant to their investigation and would like to identify and speak to her.
The woman pictured, anyone who recognises her or has information about the incident should call Northamptonshire Police on 101.
Incident number: 21000015720