“I will not be seeking any political post after my term ends,” she told a news conference in Berlin.
Merkel also said she would not seek re-election as leader of the centre-right CDU party in December. She has held the post since 2000.
The CDU was severely weakened in Sunday’s poll in the state of Hesse, the latest in a series of setbacks.
Both the CDU and its coalition partners, the Social Democrats, were 10 percentage points down on the previous poll there – even though they remain in power in Hesse.
Merkel said she took “full responsibility” for the poor performance, and made it clear she would not handpick her successor as party leader.