This was confirmed by PKR vice-president Tian Chua in a Twitterposting.
Ibrahim (right), who professed himself as among the early leaders to bring Umno into Sabah, cited endemic corruption as triggering his decision to abandon the party.
He was also former Sabah Umno treasurer.
Previously, Ibrahim's name had been bandied around when speculation first emerged that an imminent defection was pending in Sabah.
He had also been named in the blogosphere as a potential candidate for the Labuan parliamentary constituency.
Three other major Sabah figures had defected thus far, namely Beaufort MP Lajim Ukin, Tuaran MP Wilfred Bumburing and senator Maijol Mahap.
“There may be a big (political) earthquake in Sabah. The mood is very different...
“If you look at the response from Umno leaders, they are very cautious and very worried. I think the mood of the people is very high,” he said.
He also claimed that the ceremony-cum-ceramah drew a crowd of about 2,000.