The Nickelodeon star – who gave birth to her daughter Maddie in 2007 when she was just 16 – has been in an on/off relationship with the father Casey Aldridge ever since, but moved back in with her mother Lynne Spears six weeks ago.
A source told People magazine: "She did a lot of growing up but he didn't. I think she put up with more than she cared to put up with. She just had enough of it and was tired of the situation."
The 18-year-old star – the younger sister of Britney Spears – has reportedly already moved on, and is dating James Watson, 28, who owns a multimedia company in Los Angeles.
A Spears family friend said her new love interest "seems to be a nice guy".
Jamie Lynn and Casey had originally planned to marry, but in March 2009 it was reported the engagement was off.
Jamie Lynn's representatives would not comment on her split from Casey.