Sheikh Abdullah Salem Al-Sabah and Sheikh Nawaf Malek Al-Sabah were arrested for expressing political views on Twitter, the independent Kuwait Human Rights News Centre said.
Sheikh Abdullah's lawyer Al-Humaidi Al-Subaie said his client was arrested late on Wednesday and was expected to be questioned by the public prosecutor later on Friday.
The two young royals have written tweets sympathetic to the Kuwaiti opposition, which has been organising protests against an amendment to the electoral law seen as designed to produce a pro-government parliament in a snap December 1 general election.
More than 150 protesters and 24 police have been injured at demonstrations since October 21 and the opposition plans another rally on Sunday. Young royal Sheikh Meshaal al-Malek Al-Sabah was detained for a few days in July for expressing political views deemed offensive.