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.
Meghan Stapleton, a spokeswoman for the Palin family, said Johnston sees the baby whenever he wants and so does his family.
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.
The young Palin told Greta van Susteren that they intended to get married after they both finished high school.
"Eventually, we'd like to get married. We're focusing on, like, getting through school and just getting an education and stuff, getting a career going," Palin said. She also told FOX News that she and Johnston didn't find being young parents "glamorous," and they both felt having kids should wait.
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."
The Associated Press contributed to this report.