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Ms Hilton told the judge she was very sorry and that she would "pay complete attention to everything" from now on.
The socialite said she did not know her licence had been suspended when caught driving without headlights in February.
According to papers filed in Los Angeles' Superior Court, Ms Hilton was stopped by California Highway Patrol officers on 15 January and informed she was driving on a suspended licence.
The 26-year-old then reportedly signed a document acknowledging she was not allowed to drive.
I'm very sorry and from now on I'm going to pay complete attention to everything 
On 27 February, she was stopped by Los Angeles County Sheriff's Department for speeding on Sunset Boulevard with her car's headlights off and charged with violating her probation.
Although Ms Hilton maintained she was not aware her licence had been suspended, court papers said the document she signed in January was found in her car's glove compartment.
She was also accused of failing to enrol in an alcohol education programme by a court-ordered deadline.
Late entrance
Ms Hilton was sentenced to three years' probation, fined $1,150 (£582), and lost her licence at the end of January after pleading no contest to a drink driving charge in September.
Despite prosecutors saying the heiress deserved a jail sentence prior to Friday's hearing, Ms Hilton arrived at the court at least 10 minutes late.
One of the world's most photographed women, her arrival attracted a swarm of paparazzi at a courthouse whose most glamorous work usually consists of dealing with parking fines. 
Her mother, Kathy, laughed when the demand for a custodial sentence was made during the prosecution's closing argument and asked Judge Michael Sauer for his autograph.
"I'm very sorry and from now on I'm going to pay complete attention to everything," Ms Hilton told the court before she was sentenced.
"I'm sorry and I did not do it on purpose at all," she said. But the judge was unmoved.
The 26-year-old heiress wept when the ruling was announced, and her mother shouted at the prosecutor: "You're pathetic."
One of Ms Hilton's lawyers said the jail sentence was unfair.
"To sentence Paris Hilton to 45 days in jail to me was uncalled for, inappropriate and bordered on the ludicrous. I think she was singled out because she's who she is," Howard Weitzman said.
If Ms Hilton does not report to jail on 5 June she will face 90 days behind bars.
Ms Hilton, heiress to the Hilton Hotel fortune, first gained notoriety for her constant partying as a teenager.
The BBC's David Willis says the austere surrounding of the all-women's detention centre will be a far cry from the five-star luxury the hotel heiress has become used to.
What her fellow inmates will make of her, and she of them, remains to be seen, our correspondent says.
