The employee, who works at the Grey Friar in Friargate, tested positive for Covid-19 on Wednesday (October 7).
The staff member and two others who may have been in close contact with the employee are now self-isolating for 14 days.
A spokesman for Wetherspoon said: "We can confirm that one employee at the Grey Friar, Preston has notified us of having tested positive for Covid-19 on 7 October. For data protection reasons, we cannot confirm the identity of the employee who tested positive.
"In accordance with NHS guidance, the employee has been required to self-isolate for the requisite ten-day period. Public Health England has been notified of the positive result. Wetherspoon wishes the employee a full and speedy recovery.
"As a precaution, in accordance with public health guidelines, two other members of staff who may have come into close contact with the employee have been asked to self-isolate for 14 days. Close contact means being within two metres of a person who tested positive for the virus for 15 minutes or more or within one metre for one minute or more."
Wetherspoon has confirmed they have not been notified by the Environmental Health Officer, Public Health England or NHS Test and Trace of any infections among customers.
"Since an approved social distancing policy is in operation in all our pubs, we think it is unlikely that customers at the Grey Friar, Preston will need to be contacted using the Test and Trace system, but that is a matter for the public health authorities," the spokesman added.
"Since reopening, Wetherspoon has operated comprehensive social distancing and hygiene practices in all its pubs. These include; reduced capacity levels, the spacing out of tables, the installation of a number of floor screens between tables and the addition of till-surround screens to the bar.
"While it is not possible to create an entirely risk-free environment, adherence to social distancing guidelines and rigorous implementation of hygiene standards minimises the risk."
The news comes after it was announced Lancashire will be classed as a Tier 2 area of "high risk" for transmission of the virus.
That means household mixing will be banned in all indoor settings - both homes and public venues –with the latter now becoming legally enforceable.
Currently in Lancashire, mixing is illegal – except for support bubbles - in dwellings and gardens, but there is only strong advice against it elsewhere.
The rule on multiple households gathering in gardens is also partially relaxed under this tier - it is now to be allowed, but still subject to the rule of six.
The new regulations will come into force from Wednesday (October 13), subject to Parliamentary approval.