Spears' lawyers entered the plea on her behalf during a hearing in Ventura County Superior Court, north of Los Angeles.
The 44-year-old singer was not required to attend the proceeding because it was a misdemeanour case, and she was not present.
Spears, whose sometimes turbulent personal life has frequently overshadowed her showbiz career, was originally charged with a single count of driving while under the influence of drugs and alcohol after California Highway Patrol officers arrested her in March.
Police tracked down Spears and her vehicle in response to a report of a motorist who was seen driving erratically in a black BMW at high speed in Ventura County.
She was taken into custody when officers observed her exhibiting signs of impairment, the Highway Patrol said at the time.
A legal filing said the recording artist drove under the influence of a combination of alcohol and at least one drug.
It did not specify which substances authorities believe she used.
She was offered the chance to plead guilty to a lesser charge of reckless driving involving alcohol and drugs, a common resolution for someone who has no prior DUI history, did not crash and has shown a willingness to undergo treatment, according to the Ventura County District Attorney's Office.
The reckless driving alternative carries a sentence of 12 months on unsupervised probation.
Had she been convicted of the more serious DUI offence, she would have been placed on probation for three years.
If Spears re-offends while on probation, she would face another DUI charge with tougher penalties that could potentially include a year in jail and five years of probation, District Attorney Erik Nasarenko told reporters after the hearing.
There are no restrictions on her driving privileges other than a strict prohibition on getting behind the wheel with any level of intoxicants in her body.
And if pulled over for any reason, she must submit to a blood-alcohol test and search of her vehicle.
As part of the plea deal, Spears also must complete a 30-hour program of alcohol-abuse education and continue her regimen of individual mental health therapy sessions once a week, along with twice-monthly visits with a psychiatrist, court Commissioner Matthew Nemerson ordered.
A representative for the Grammy-winning performer said in April that Spears had voluntarily checked into a rehabilitation facility after the DUI arrest.
