Former “American Idol” contestant Randy Madden is facing some serious charges … he’s been arrested for allegedly having sex with someone who is underage … TMZ has learned.

Madden, who appeared on Season 8 of the singing competition show, is facing six felony charges … including oral copulation of a person under 18 and unlawful sexual intercourse.

He’s also charged with anal and genital penetration by foreign object, sending harmful matter, luring, and dissuading a witness from reporting a crime.

It’s unclear how old the alleged victim is. Madden is 45.

Court documents show the charges stem from at least one alleged sexual encounter between July and September of this year… he was charged Monday, pleaded not guilty to all counts and was taken into custody.

Court officials tell us Madden posted $100,000 bond Tuesday morning, and was released from jail.

‘Idol’ fans might remember Madden from Season 8, when he auditioned in Phoenix, and sang “Livin’ On a Prayer” by Bon Jovi. All four judges gave Madden a “No” vote, however, and he was eliminated from competition.