Swansea City say a Wycombe fan was ejected from the Liberty Stadium after entering the ground during Saturday's Championship game between the clubs.
A man was spotted in the North Stand before being met by stewards.
Fans have not been allowed at Swansea home games since March 2020 because of the coronavirus pandemic.
"While this is very much an isolated incident, the club will continue to monitor all areas of security on a matchday," Swansea said in a statement.
"A Wycombe Wanderers supporter gained entry to the Liberty Stadium during the game before being apprehended by stadium security staff.
"South Wales Police swiftly dealt with the person involved and he was ejected from the premises."
Promotion-hopefuls Swansea fought back to draw 2-2 with the visitors, but ended the day with an injury worry over striker Andre Ayew.