It will offer drop-in Covid-19 vaccinations to anyone eligible for vaccination.
This initiative has been designed to allow those working in retail and hospitality quick and easy access to the vaccine, without the need to take any significant amount of time away from work.
It will also target shoppers and those visiting the town centre on a day out.
The unit will complement the drop-in clinics that are already running at mass vaccination sites across the Lothians, including at Gorebridge Leisure Centre.
The unit will operate from 11am until 7pm.
The unit has already been out and about in Edinburgh and the Lothians.
Anyone who has not yet received an invitation to one of the existing fixed sites or would like to rearrange an existing appointment can continue to use the self-registration portal.
The St Andrew Street car park will be closed for parking from 8pm on Tuesday to 8pm on Wednesday.