Sheikh Abdullah Salem Al-Sabah was released late on Friday, while Sheikh Nawaf Malek Al-Sabah was freed on Saturday afternoon, AFP reported.
"I asked them (police) to refer me to the public prosecution to defend myself from the horrifying accusations, but they insisted that I sign a pledge and they released me," late Friday, Sheikh Abdullah said on Twitter.
Sheikh Nawaf's lawyer Khaled al-Suwaifan said Saturday that his client was released but provided no details.
Sheikh Abdullah said he was questioned by the secret service police on accusations of insulting Emir Sheikh Sabah al-Ahmad Al-Sabah and instigating against the regime.
He is the grandson of the emir's half-brother, the late Sheikh Abdullah al-Ahmad Al-Sabah.
The two young royals have written tweets sympathetic to the Kuwaiti opposition, which has been organizing protests against an amendment to the electoral law seen as a ploy to produce a pro-government parliament in a snap December 1 general election.
More than 150 protesters and 24 police have been injured in demonstrations since October 21 and the opposition plans another rally on Sunday.
Young royal Sheikh Mashaal al-Malek Al-Sabah was detained for a few days in July for expressing political views deemed offensive.