Opposition politicians Jamaan al-Harbash and Waleed al-Tabtabai forced their way into parliament in 2011 after lawmakers were prevented from questioning then prime minister Sheikh Nasser al-Mohammad al-Sabah about corruption allegations.
They were tried and sentenced for that last year but had already left Kuwait. In October, parliament defied the government and voted to keep on the pair, who have a large political following.
The court ruled that the article requiring a vote to strip legislators of membership was "unconstitutional", state news agency KUNA reported.
There was no immediate reaction to the ruling from the opposition or either of the two lawmakers, whom opposition sources say is in Turkey. Kuwait's highest court in July reduced their jail terms to 3-1/2 years from an original five.
U.S. ally and OPEC member Kuwait avoided mass Arab Spring-style unrest, though citizens held protests in 2012 after Emir Sheikh Sabah Al-Ahmad Al-Jaber al-Sabah changed the electoral law.
While Kuwait allows more freedom of speech than some other Gulf states, the emir has the last say in state affairs.
(With inputs from agencies.)