The gold and silver pieces that the 23-year-old reportedly dug up without permission are thought be from the 5th century, and therefore extremely valuable.
Prosecutors accuse him of embezzlement, saying that he failed to report the find in a timely manner.
The accused argues that he did eventually report his discovery after a few months, but the prosecution say that it was too late.
The accused said before the court that he hadn't realised the considerable value of the treasure, and just wanted to clarify the nature of his findings.
"I wanted to act as a kind of researcher, show ambition, and document the discovery as best as possible before I handed it in," the 23-year-old said. When he complained that he was being criminalized, a number of people present in the room began to applaud.
He explained how he had found the treasure in May 2013, while exploring a wooded area in Rülzheim, near Karlsruhe, with his metal detector - but without permission from the landowner.
The prosecutor accused the young man of knowing full well the value of his discovery and intending to keep it for himself.
A judgement may not be reached on Wednesday, but a guilty verdict could see the man face a hefty fine or up to three years in prison.
Archaeologists celebrated the sensational discovery over a year ago.
The excavated pieces are the only ones of their kind to have been found in Germany to date, and their total material value is between €425,000 and €575,000.
Last year archaeologists put the findings on display in Mainz. Alongside the gold and silver coins were a ceremonial garment, a silver plate, a silver bowl and golden chair.
The treasure is likely to have belonged to a high official or prince of the 5th century, experts say.
During the period Germanic tribes would flock to the already-crumbling Roman Empire in order to plunder anything valuable.
by Matty Edwards