Image copyright: Dan Ellis.
The Berkshire town has one of the highest numbers of people infected with HIV in the UK outside London and has nearly double average UK rate.
It is estimated 270 people in Berkshire will be unaware they have HIV.
Thames Valley Positive Support is running extra clinics in various locations in Berkshire all week.
The bus, run by the Florey Clinic, is one of few mobile clinics on wheels in the country and offers free tests, with results taking about two weeks.
Reading has an HIV rate of 3.7 per thousand population, aged between 15 and 59, compared with the UK average in 2014 of 1.9.
In the whole of Berkshire more than 1,000 people have been diagnosed.
According to Reading Borough Council, estimates show 277 people in the county would be unaware they have the virus.
Graeme Hoskin, Reading Council's lead member for health, said: "Unfortunately, Reading has a relatively high HIV rate, but the sooner it is diagnosed, the sooner appropriate care can begin."
The bus is outside Reading Station until Saturday as part of HIV Testing Awareness Week.