Cornwall Live said its journalists were told they could not broadcast the PM's visit via Facebook Live.
The publication, owned by Trinity Mirror, took to social media to vent its frustrations.
The Conservatives said the reporters' "last minute" request to film had "not been possible" because a pooled resource had already been agreed.
Reporters and broadcasters are traditionally given a pooled resource for video, stills and text at such events.
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Cornwall Live sent two reporters and a photographer to cover the prime minister's visit to Helston.
The reporters published a photo of the room they were held in while the PM was interviewed by other media.
Editor Jacqui Merrington said they were "a lot more" than a local newspaper.
"When we arrived... we were told we were not allowed to film anything because we were invited as print media, which seems a bit 20th Century really."
She added: "We are digital media and that is the case for most local media nowadays and to be restricted on that basis seems very archaic."
A spokesman for the Conservatives said Cornwall Live only made a request to film on the morning of the visit.
"One media organisation's last minute request to add a camera to a pre-arranged pool of broadcast cameras was not possible this morning.
"The organisation's journalists did interview the prime minister and their photographer accompanied [her] on a factory tour."
Website reporters covering Theresa May's visit to Cornwall have complained they were not allowed to film her.