Celebrity photographer Markus Klinko claims he dated the Drag Me to Hell star in 2007, two years after she married Ross.
He took the stand as the prosecution's star witness when the case went to trial on Wednesday, telling jurors Romero confessed to him that she is in a marriage of convenience with Ross.
Prosecutors at Los Angeles Federal Court contend Ross was paid $5,000 (£3,330) to wed the actress so she could obtain permanent residency in the U.S.
The defendants, both 28, have pleaded not guilty to marriage fraud and making false statements. They face up to five years behind bars if found guilty.