BERLIN — Chancellor Angela Merkel of Germany, under siege domestically but widely seen as a pillar of Western liberalism, announced on Sunday that she will seek a fourth term next year.

Slightly hoarse but clearly determined after consulting leaders of her conservative Christian Democratic party, Ms. Merkel said the decision to seek a fourth term was “anything but trivial,” for her country, for her party and for herself.

A scientist with a low-key manner, Ms. Merkel rejected the idea that, after the election of Donald J. Trump as president in the United States, she had a lone role in keeping Western liberalism alive. “That is grotesque, even almost absurd,” she told reporters.

But she also said that the campaign ahead of the German elections in fall 2017 would be unlike any other she has fought in an increasingly polarized country. She faces stronger challenges on the right and left, while the war in Syria, the arrival of large numbers of migrants and the continuing euro crisis tear at Germany and place new demands on its people.