Johnston, 19, told The Associated Press that he and 18-year-old Bristol Palin mutually decided "a while ago" to end their relationship. He declined to elaborate as he stood outside his family's home in Wasilla, about 40 miles north of Anchorage.
He also said some details of the breakup, rumors of which had been swirling on the Internet, were inaccurate.
Bristol Palin said in a statement that she was devastated about a report on Star magazine's Web site that quoted Levi's sister, Mercede, as saying Bristol "makes it nearly impossible" to visit the teenagers' infant son, Tripp. The baby was born Dec. 27.
"Unfortunately, my family has seen many people say and do many things to `cash in' on the Palin name," said the statement, which was issued through the governor's political action committee. "Sometimes that greed clouds good judgment and the truth."
Sarah Palin revealed her daughter's pregnancy just days after being named John McCain's running mate on the Republican presidential ticket. She had said in December that her daughter and Johnston "are committed to accomplish what millions of other young parents have accomplished, to provide a loving and secure environment for their child."
In an interview that aired on Fox News last month, Bristol Palin said her fiance saw the baby every day and described him as a "hands-on" dad.
Johnston and Palin had said they were considering a summer wedding.
"We both love each other," he told the AP in October. "We both want to marry each other. And that's what we are going to do."