Enbridge spokesman Graham White says that a valve that is part of their line 9 pipeline in Hamilton was tampered with on Monday.
White says the line was shut down briefly overnight while maintenance workers inspected the pipeline.
He says Enbridge's control centre detected a valve closure on Line 9 just north of Hamilton at 1:15 a.m. ET Monday.
The company contacted authorities and restarted the line about four hours later.
White says there was no impact on deliveries to clients.
The latest case of pipeline sabotage follows two similar incidents in Ontario and one in Quebec over the past month or so — none of which caused injury or significant disruption.
- Activists sabotage Enbridge pipeline near Cambridge, Ont.
- Hamilton raises concerns about Enbridge Line 10 expansion
A post on Earth First Newswire, an environmental activist website, described the latest action.
"Our hearts were bursting with love and cheer after hearing of the many times our friends have courageously shut down pipelines in recent months," it said.
"So, in the early hours of January 25, 2016 we found our own courage and took action against Enbridge and their Line 9. Slipping in to a valve station located on traditional Haudenosaunee Territory (in Hamilton, Ontario) we successfully operated an electronic valve to shut off the flow of tarsands crude. A Line 7 valve, also an Enbridge tarsands pipeline, was also tampered with and closed part way. We then disappeared back into the night."
White has said previously that Enbridge has boosted its security measures.
"These valve tampering incidents had the potential to cause real harm to the public and the environment as well as to the protesters themselves."