Himpunan Hijau chairperson Wong Tack was deported from Sarawak after arriving in Kuching this evening.
Wong Tack, who arrived at Kuching International Airport at around 5pm, was informed by immigration officers that he will not be allowed into the state.
He told Malaysiakini that no reason was given to him apart that it was an "order from above".
Wong said he was in Sarawak to highlight environmental issues to voters and politicians alike, adding that he was not invited there by any political party.
He expressed shock at being barred from entering the state.
"This shows that Sarawak Chief Minister Adenan Satem is not sincere in bringing reforms," he said.
Wong Tack was then put on the next flight out back to Kota Kinabalu and departed at around 8pm.
He has previously contested in the 13th general election under DAP for the constituency of Bentong in Pahang.
He joins a list of several opposition politicians and NGO leaders to have been barred from Sarawak ahead of the May 7 elections.