UFO investigators are now calling for more witnesses to come forward after the shock report was made to a specialist organisation.
The UFO sighting allegedly happened last Tuesday in the Gulf of Mexico about 80 miles southeast of New Orleans, and reported two days later.
The report was made to the US-based National UFO Reporting Center (NUFORC), which keeps a database of sightings and investigates cases.
One unnamed crew member, who was the chief engineer of the OSV, said the UFO was five times bugger than the 240 feet oil rig supply ship they were on.
The report said: “At close to 7pm on March 21, just before dusk, myself and four of the crew members aboard our vessel saw a craft that appeared to be five times our 240 feet vessel in length.
“My line of sight was about 1/4 mile from our vessel.
“There was a rig behind the craft about a half a mile away.
“I used this to help gauge the size of the craft.”
He said it was visible for about 40 seconds.
He added: “The craft rose up out of the water about 40 feet, but no water was dripping from the craft.
“Within a split second the craft disappeared at a 30 degree angle into the sky.”
He estimated it travelled faster than the sped of light entering a room, and within second was gone.
He added: “I can say for sure that the craft was dark coloured, oval in shape and made no sound whatsoever.”
He said with so many rigs in the area there must have been other witnesses.
NUFORC is urging anyone else to get in contact about the “significant sightings.”
NUFORC investigators spoke with the witness by phone and have found him “legitimate.”
A NUFORC spokesman said: “We spoke via telephone with this witness, and he seemed to us to be unusually sober-minded.
“We suspect that he is a very capable, and very reliable, witness. He estimates that upwards of perhaps 50 people, who were aboard nearby vessels, may have witnessed the event, as well.
“We would urge those other witnesses to submit reports of what they had witnessed.”
Many conspiracy theorists claim that aliens do visit Earth and have secret undersea bases for their flying saucers.
No firm evidence for this being the case has come forward.