The Clark County district attorney's office filed a criminal complaint Friday alleging the rising singer-songwriter had 2.6 grams of cocaine when he was arrested Sept. 19 after being detained by a hotel security guard.
Mars, whose real name is Peter Hernandez, is due in court Nov. 18 on the possession of a controlled substance charge. If convicted, he faces up to four years in prison and a $5,000 fine.
"Just the Way You Are" is No. 1 on the Billboard Hot 100 chart.
Hernandez, 24, was in Las Vegas for a performance at the Hard Rock Hotel & Casino's Wasted Space nightclub.
After the show, a bathroom attendant told police Hernandez was taking a long time in a stall with a bag of a white, powdered substance, according to the arrest report. The attendant alerted a security guard, who confronted Hernandez as he left the stall. The security guard said he asked Hernandez to hand over any narcotics he had, and Hernandez removed a bag of cocaine from his left-front jeans pocket, the report said.
Hernandez was taken to a holding cell, where he was questioned by an arresting officer. When asked what was going on, Hernandez replied: "Can I speak to you honestly, sir?" He said he acted foolishly and had never used drugs before, the arrest report said.
Hernandez sang the hooks on B.o.B's "Nothin' on You" and Travie McCoy's "Billionaire" - both chart-toppers - and co-wrote and produced Cee-Lo Green's latest expletive-laden single.
His label, Elektra Records, released a brief statement Friday: "We congratulate Bruno Mars on his chart-topping success, and provide him with our full love and support."
Hernandez's debut album, "Doo-Wops & Hooligans," is scheduled for release Tuesday.