In a palace statement, he said such an act would "defy the nation's culture".
Princess Ubolratana Mahidol, 67, has been nominated as a candidate for a party allied to divisive former PM Thaksin Shinawatra.
Such a move would break with the tradition of the Thai royal family publicly staying out of politics.
Analysts say the king's intervention is likely to lead to the election commission disqualifying her from the 24 March election.
The vote is being closely watched as the first chance for Thailand to return to democracy after five years under military rule.