Aaron Pace, 36, of North Philadelphia, pleaded guilty to third-degree murder in court on Wednesday morning and was sentenced to 17 1/2 to 35 years in prison.
The incident happened on February 2, 2010.
Police say Pace, a neighbor, was babysitting the 20-month-old baby boy, Suliaman Kirkland, who was crying while in the bathtub.
"He was so full of life, even at 20 months. He fought hard until he couldn't fight no more. I'm so proud of my son," said his mother, Quintesha Kirkland, after court on Wednesday.
When the child wouldn't stop crying, investigators said Pace rubbed the baby's skin with a rag soaked in Drano, causing severe burns.
Prosecutors said Pace waited to take the baby to the hospital and was never forthcoming about what happened.
The baby died in the hospital five days later.
"He rubbed Drano all over this little boy who was 20 months old when he was fussing in the bathtub because he got mad at him. The little boy suffered horrible injuries all over his face, his neck, his body and he stayed alive for five days with these horrible burns," Assistant District Attorney Christine Wechsler said.
Prosecutors said Pace confessed after he was arrested by homicide detectives.
Pace said he understood what he was doing by pleading guilty, though defense attorney Fred Goodman said his client's IQ was between 57 and 68.
Members of Pace's family were ejected from the courtroom for swearing and threatening the Kirkland family.
For their part, members of the Kirkland family said they were not happy with the outcome, saying the sentence was too lenient.
"My son's in Mt. Moriah Cemetery," said Kirkland. "That's not enough."
The prosecutor, however, said she was happy with the outcome, saying this spares the family a lengthy trial.