The incident occurred last weekend after a member of the audience appeared to throw water at the performer.
A video of the WAP star taking matters into her own hands at Drai's Beachclub was widely shared on social media at the time.
But she will now face no charges due to "insufficient evidence", police said.
"After a thorough review of this case and with the consultation from the Clark County District Attorney's Office, this case has been closed as having insufficient evidence," the Las Vegas Metropolitan Police Department said in a statement shared the PA news agency.
"No charges will be filed in relation to this case."
Lawyers for the the Grammy Award-winner responded by thanking the police for their "diligent and prompt resolution of this matter", in a statement reported by multiple outlets.
In the footage, caught on camera from multiple angles by several fans, security staff are seen surrounding the perpetrator at the front of the crowd while recovering the rapper's microphone.
Footage from earlier in the concert showed Cardi B asking a different member of the audience to spray her from behind, as she turned her back to the crowd.
Later in the show, the 30-year-old explained the earlier spray had been done on her back and under her own instruction, whereas she objected to the other member of the crowd unexpectedly throwing water which hit her face.
Las Vegas police later confirmed that a show attendee had come forward to "report a battery", but have now concluded that no crime was committed.
The microphone in question is now reportedly being auctioned off on eBay - reaching more than $99,000 so far - by the owner of an audio company.
Cardi B is best known for hits such as I Like It, Up, Bodak Yellow and WAP - her sexually-charged bass-heavy rap duet with Megan Thee Stallion.
She is the latest performer to have had items thrown at them while onstage in recent months, following similar incidents involving Harry Styles, Pink and Bebe Rexha.
Rexha was taken to hospital after being hit by a phone while performing. The man charged with the incident said he thought "it would be funny".
Styles was hit in the eye with a sweet at a gig in November and more recently Pink looked stunned when a bag of human ashes was thrown on stage.
Sweet but Psycho singer Ava Max has also been slapped on stage of late, while somebody else threw a bracelet at country singer Kelsea Ballerini.
Adele recently joked with her Las Vegas audience that she would "kill" anyone who tried to chuck something at her.