Khalid al-Hajri was dismissed for the second time earlier this week, only days after Kuwait National Petroleum Co (KNPC) reinstated him, the group said in a statement on Friday
He was already sacked in early November after allegedly refusing to bow to pressure to halt the group's activities. Hajri was reinstated early January.
Hajri told French news agency AFP that he would raise the issue with members of Kuwait's outspoken parliament and was planning to take KNPC to court for violating the emirate's constitution.
Greenline, an environmental group working on the same lines as the Greens in Europe, was established in September 2001 as a voluntary organisation.
Hajri said the group had exposed that KNPC buried chemical waste at two of the three refineries in Kuwait and also in the desert. It had also provided documented evidence that liquid waste was being dumped in the sea.
Oil-rich Kuwait produces about two million barrels of crude a day and the refineries have a total daily capacity of more than 900,000 barrels, the bulk of it for export.
Greenline condemned the sacking as a "human rights violation," and an attempt to force the group to stop dealing with environmental issues.