He had originally denied murdering a newborn son and enslaving his daughter in a cellar underneath his house, but reversed his plea on Wednesday, after watching Elisabeth's video testimony.
The retired engineer was found guilty of murder because he failed to seek medical help for the infant, despite knowing the boy was in danger of dying.
The prosecution had called for the "maximum sentence" to be given in a closing statement.
Christiane Burkheiser, the chief prosecutor, said: "It was murder by neglect and that demands the maximum sentence".
Fritzl made an expression of regret before the verdict was delivered.
"I'm sorry from the bottom of my heart. Unfortunately, I can't change anything now," he said.
But Eva Plaz, Elisabeth's lawyer, said Fritzl did not have "a trace of remorse".
"Don't believe one word from the defendant. The defendant wanted to be master over life and death," she said.
Adelheid Kastner, a psychiatrist, told the court on Wednesday that Fritzl was emotionally stunted and could continue to pose a threat.
"He is aware, he says so himself, that he has an evil side.
"He is aware that he was born to rape. He has that partly under control. But as soon as he loosens his grip, everything erupts out."
|Rudolf Mayer, Fritzl's lawyer, said he was very surprised at his client's change in plea [EPA]
Fritzl built a soundproofed cellar with a reinforced door under his home in Amstetten.
"He shut [Elisabeth] away in the cellar and made her totally dependent on him, forcing her into sexual acts and treating her as if she was his own property," his charge sheet said.
Fritzl's daughter and her six children, three of whom were shut away from birth, are now living in a secret location under new identities.
The case came to light when one of the three children who had never seen sunlight, 19-year-old Kerstin, fell seriously ill and was taken to hospital by Fritzl.
In comments via his lawyer last year, Fritzl said he had lived a "second life" in the cellar complex, watching adventure videos with the children and bringing flowers for Elisabeth, who cooked dinner.
Three of the children born in the cellar were raised above ground by Fritzl and his wife after he pretended Elisabeth had abandoned them.
Police say that Fritzl's wife did not know of his actions.