Caylee Anthony, 2, has been missing since June in a case that has received national attention.
The announcement marks the end of a six-month search for the 2-year-old.
"It is with regret that I'm here to inform you that the skeletal remains found on December 1 are those of the missing toddler," Orange County Medical Examiner Dr. Jan Garavaglia said.
She said the cause of death was homicide, but she could not determine how Caylee was killed.
Casey Anthony, 22, faces charges including first-degree murder in the June disappearance of her daughter. Remains described as being those of a small child were found last week a half-mile from Casey Anthony's parents' home, in the area where a meter reader first directed police.
At Friday's news conference, police will identify the meter reader who, they said Thursday, called the department four months ago, directing them to the site of the remains three times in August.
At a Thursday news conference, Capt. Angelo Nieves, a Sheriff's Department commander, said investigators were looking into whether the tips, called in August 11, 12 and 13, were properly followed up.
In one of those phone calls, the meter reader reported seeing a gray bag on the side of the road, Nieves said. On August 13, a deputy responded to the site and did a "cursory search" but found nothing, Nieves said.
Nieves said police were getting more information from the tipster and the deputy who responded to the tips. He said the department was investigating the "thoroughness" of the deputy's response but would not identify the deputy.
The meter reader "is not a suspect," Nieves said. "He is a credible witness."
Nieves' latest announcement is raising questions about whether police missed several chances to find remains believed to be Caylee's.
The meter reader is not the only one, or the first, to have pointed police toward the site containing the remains.
KioMarie Cruz, Casey Anthony's childhood friend, also told police to investigate the same wooded area near Hidden Oaks Elementary School a month before the meter reader, according to CNN affiliate WFTV.
In an interview with detectives, according to WFTV, Cruz said that she and Anthony "pretty much used to hang out there most of our time," would "snack on food for hours" and went there to "get away from our parents."
The Sheriff's Department followed up on that tip, but the wooded area was covered in floodwaters, preventing a search. Nieves said the water may have been present at the time of the meter reader's tips as well.
Nieves also said Thursday that searchers combing the site after the skull's discovery had found "significant skeletal remains" consistent with those of a small child on the outer perimeter of the search area.
The area will be enlarged, and processing and searching of the site will continue, probably into the weekend, he said.
Some of the remains have been sent to the FBI lab in Quantico, Virginia, in an effort to identify them. Authorities have said the remains are believed to be Caylee's, but an identification is pending.
Sheriff's spokesman Carlos Padilla said last week that authorities believe the remains are Caylee's for three reasons: No other children have been reported missing in the area; the remains are consistent with those of a child of Caylee's age; and the remains were found near the home of the grandparents, where the 2-year-old and her mother were living just before Caylee disappeared.
FBI spokesman Richard Kolko said Monday that he did not know when tests would be complete, but an attorney for Anthony's parents said the FBI is likely to have results "within the next week."
Casey Anthony could face a sentence of life in prison if convicted. Prosecutors said this month that they would not seek the death penalty.