State Rep. Thad Viers was fined $500. His lawyer said he was "under the influence of love."
The misdemeanor unlawful communication charge carried a maximum sentence of 30 days in jail. Mr. Viers, 29, left court immediately but later released a statement saying he was ashamed of his behavior on a day that he had hoped to reconcile with his wife, only to learn she was dating someone else.
"I and those around me lost our tempers and exercised poor judgment," said Mr. Viers, a Republican who represents the Myrtle Beach area. "In most of our lives, we can point to a time where as it relates to matters of the heart our emotions override our good sense. These events represent that for me and I look forward to moving on with my life."
The man dating Mr. Viers' estranged wife, James Zeigler, told police last fall he received several threatening phone calls from Mr. Viers.
Mr. Zeigler said Viers identified himself in the calls, which also threatened his mother, according to an incident report from the Richland County Sheriff's Office.
Mr. Zeigler told police he also received calls from unidentified people threatening to harm him on Mr. Viers' behalf. Mr. Viers has said friends could have placed the calls from his phone without his knowledge.
Mr. Viers's attorney, who is also a state legislator, said his client was "under the influence of love" when he made the calls. State Rep. Todd Rutherford said the accusations were overblown.
"This was an attempt of a paramour of my client's wife to get back at my client," said Mr. Rutherford, a Democrat who represents Columbia.
Mr. Viers and his estranged wife, who is pregnant with Mr. Zeigler's child, are in the process of getting a divorce, Mr. Rutherford said.
Mr. Zeigler declined to say Thursday if he and Mr. Viers' wife were still together.
Mr. Viers graduated from the University of South Carolina law school this spring. He was first elected to the House in 2002.
Mr. Viers applied for a pretrial intervention program that would have erased the charge from his record if he completed community service and other requirements, but he was denied acceptance in April.