An Indian national has been sentenced to nine years in prison in the US for a brazen sexual assault of a woman on an airplane while his wife sat next to him.

Prabhu Ramamoorthy was sentenced on Thursday in a Detroit court, the US Justice Department said. The 35-year-old, who is in the US on a work visa, will be deported to India after completing his sentence, it said.

Prosecutors said Ramamoorthy was sitting in a middle seat on a January overnight flight from Las Vegas to Detroit, when he digitally penetrated a sleeping woman beside him as his wife sat on his other side.

The woman was woken by the assault to find her pants unbuttoned and unzipped, and sought help from flight attendants.

Crew members told federal investigators the victim was crying as she reported the incident 40 minutes before the plane's landing, according to the Washington Post.

Ramamoorthy was arrested after the plane touched down.

“Everyone has the right to be secure and safe when they travel on airplanes,” US Attorney Matthew Schneider said.

“We will not tolerate the behaviour of anyone who takes advantage of victims who are in a vulnerable position. We appreciate the victim in this case for her courage to speak out.”

A jury convicted Ramamoorthy in August after a five-day trial. He had faced up to life in prison.

Ramamoorthy cried as he left the courtroom on Thursday, as his victim looked on, sitting quietly with a comfort dog, the Detroit Free Press reported.