Explicit cartoon images were displayed publicly on a large advertising touchscreen near Aberystwyth train station after a "fault" allowed someone to use the screen to go online.
The screen's owner, Shaun Jones, said it was "horrendous timing" as the cause of the fault allowing web searches had been due to be removed.
He said he removed the images remotely within a few minutes of them appearing.
Dyfed-Powys Police said it is investigating.
It happened on one of three screens promoting businesses and travel advice.
Mr Jones said a local police and community support officer had alerted him to the incident on Sunday.
"We take pride in these screens. It was an act of vandalism," said Mr Jones.
He said the problem with a faulty player generated an onscreen keyboard people could use to get online and conduct web searches.
He said that fixes had already been applied to the two other touchscreens in the town.
Dyfed-Powys Police said it received a report that pornographic images were displaying on an interactive screen at Alexandra Road in the town at about 17:30 GMT on Sunday.
"The screen was quickly covered and engineers were called to switch the screen off," the force added.