Palace spokesman Sven Gjeruldsen said the 35-year-old princess was unwell, but declined to give details. "I cannot say anything else but that the princess is sick," he said.
The announcement followed intensive media speculation last week about her involvement in starting an alternative school to teach people how to get in touch with angels.
On the Web site of the Astarte Education school, Martha Louise says she has learned how to communicate with horses and to talk to angels.
The palace declined to comment on the princess' involvement in the school. "When it comes to the princess' professional doings we don't comment," Gjeruldsen said.
He also declined to say how long the princess' sick leave would last.
"She has official duties scheduled for Aug. 11, and there is nothing to say that she won't be able to take part in that," Gjeruldsen said.
Martha Louise is fourth in line to the throne after her nephew, Prince Sverre Magnus. She was married in 2002 to Ari Behn, a Norwegian writer.