Peterson, 59, a retired Bolingbrook, Ill., police sergeant, had faced 20 to 60 years in prison for the 2004 slaying. Peterson's fourth wife, Stacy, disappeared in 2007 and remains missing.
Will County Judge Edward Burmila said he did not impose the maximum sentence in acknowledgment of Peterson's military and police service, the Chicago Tribune reported.
Moments before the sentence was handed down, Peterson screamed, "I did not kill Kathleen!"
He told the judge he was convicted by "rumors, gossip, outrageous lies and, most importantly, unreliable hearsay."
"I don't deserve this," he told Burmila. "I don't deserve this."
State's Attorney James Glasgow called Peterson's outburst a "shrill, kind of feminine screech."
Burmila earlier rejected arguments from Peterson's legal team that his former lead attorney Joel Brodsky botched his defense, leading to his conviction last fall of first-degree murder, the Chicago Tribune reported.
"It was clear to the court from the very beginning that Mr. Brodsky was out of his depth," said Burmila, but he said Peterson had five other attorneys on his team and "each of these attorneys brought something to the table."
At issue was Brodsky's decision to call divorce lawyer Harry Smith to the stand. Smith testified that Stacy Peterson told him Peterson said he had killed Savio and wanted to know if that statement could be used in a divorce proceeding.