Mike McKay told his employers, in an email made public overnight, that he had observed the plane burning at high altitude.
"I believe I saw the Malaysian Airlines plane come down. The timing is right," he said.
"I tried to contact the Malaysian and Vietnam officials several days ago. But I do not know if the message has been received."
Mr McKay said he was on the oil rig Songa Mercur, off south east Vietnam.
When he observed the plane it appeared to be in one piece, he said.
"From when I first saw the burning (plane) until the flames went out (still at high altitude) was 10 to 15 seconds," he wrote.
Several online news outlets have posted the letter, and say the man's employer has confirmed it's authentic.
ABC reported that Vietnam officials confirmed they had received the email but found no wreckage in the area specified.
