The 19 year old called off their engagement and split from Johnston last year (09) after giving birth to their son Tripp in December 2008.
The pair has since been locked in a bitter custody battle, but they reignited their relationship after a meeting earlier this year (10).
Palin and Johnston have now announced they are to wed - but Sarah Palin has been kept in the dark about the impending nuptials because Bristol is "scared" about her reaction.
She tells US Weekly magazine, "We got engaged two weeks ago. It felt right, even though we don't have the approval of our parents... It is intimidating and scary just to think about what her (Sarah Palin's) reaction is going to be. Hopefully she will jump on board."
Speaking about rekindling the romance she adds, "When he left that night (after the custody meeting), we didn't hug or kiss, but I was thinking how different it was. He texted me: 'I miss you. I love you. I want to be with you again'... I was in shock."