A 48-year-old Florida polo mogul — about to go on trial for DUI manslaughter — adopted his 42-year-old girlfriend in a move that could shield much of his fortune from a wrongful-death lawsuit.

Billionaire John Goodman’s adoption of Heather Hutchins, his girlfriend since 2009, is “surreal” and moves the case into “a legal twilight zone,” said Palm Beach County Circuit Court Judge Glenn Kelley.

Goodman, founder of the International Polo Club and heir to a Texas air-conditioning fortune, was driving his $200,000 Bentley in February 2010 when he ran a stop sign in Wellington, Fla., and struck the Hyundai of Scott Patrick Wilson, authorities said.

Wilson, 23, drowned when his car flipped over into a canal. Investigators said Goodman’s blood-alcohol level was twice the legal limit.

Goodman goes to trial for DUI manslaughter and other charges on March 6 and faces up to 30 years in prison if convicted.

But he also faces a civil trial, starting three weeks later, in a wrongful-death suit brought by William and Lili Wilson, the victim’s parents.

Judge Kelley has ruled that a trust fund Goodman set up for his two children was off-limits if a jury awards money to the Wilsons.

Instead, Goodman stood to lose much of his personal assets, which includes his airplane, a yacht and a stable of polo horses. His family business was sold seven years ago for more than $1 billion.

But the Wilsons claimed in court papers last week that Goodman pulled a fast one last Oct. 13 — by finalizing a legal adoption of Hutchins that makes her entitled to one-third of the trust. The trust is believed to be worth at least $200 million.

Florida adoption attorney Charlotte Danciu said Goodman’s move was “both awful and brilliant.”

But the Wilsons called it a “game changer” and asked Kelley to consider the trust to be part of Goodman’s assets and subject to damages awarded by a jury.

Goodman’s attorney, Daniel Bachi, said the adoption “has nothing to do with the lawsuit currently pending against him.”

He said it was it “was done for estate-planning purposes and to ensure protection of both his and her minor children and the stability of all the family investments.”

Adult adoptions are usually done for the purposes of estate planning. But adopting a girlfriend appears to be a new wrinkle.

Kelley called the adoption “unprecedented” — but didn’t say it was illegal.