"Ayyad al-Harbi was sentenced to two years in jail," director of the Kuwait Society for Human Rights, Mohammad al-Humaidi, said on his Twitter account.
According to the ruling, Harbi must start serving the sentence immediately, even before the outcome of his challenges in the appeals and supreme courts are known, Humaidi said.
On Sunday, the same court handed an identical sentence to Rashed al-Enezi, who faced similar charges. Enezi was arrested from the courtroom and sent to jail.
In another decision, the court on Monday acquitted opposition activist Osama al-Munawer, a member of the scrapped 2012 parliament, on charges of abusing the Emir's authority and undermining his status.
Munawer was briefly arrested and interrogated after delivering a speech at a public rally on October 13. He was later charged with making remarks offensive to the Emir.
U.S. State Department spokeswoman Victoria Nuland said Washington had already raised its concern about the sentences.
"We call on the government of Kuwait to adhere to its tradition of respect for freedom of assembly, association and expression," Nuland told journalists, adding such rights were enshrined in international treaties.
"You know how strongly we feel about locking people up for their use of Twitter," she added.
Criticizing the Emir is illegal in Kuwait and is considered worthy of a state security charge. Under the law, people who are convicted of the offence face a jail term of up to five years.
Enezi and Harbi, both in their 20s, are the first to be sentenced among dozens of tweeters, activists and ex-opposition lawmakers who have been charged with similar offences since the government began a clampdown ahead of elections held on December 1.
The opposition has been staging regular demonstrations in protest at an amendment of the electoral law and the subsequent holding of December's parliamentary vote on the basis of the amended legislation.
Hundreds of opposition supporters demonstrated late Sunday to demand the dissolution of parliament, which they say is illegitimate, and the scrapping of the controversial amendment.
Riot police fired tear gas and stun grenades to disperse the small crowd and in the process arrested more than 70 protesters, according to Humaidi. Seventeen activists remained in detention on Monday, he said.
Humaidi told AFP on Sunday that more than 200 opposition activists, including former lawmakers, face trial on a variety of charges, mainly criticizing the emir who is protected against criticism in the constitution.
Among these are around 25 young tweeters who were arrested, interrogated and then freed on bail on charges of insulting the Emir.
A similar incident happened last June, when a court sentenced a 26-year-old Kuwaiti to 10 years in prison after ruling that he had endangered state security by insulting the Prophet Mohammed, his wife Ayesha, and his companions on social media, his lawyer Khaled al-Shatti said.
The accused, Hamad al-Naqi, was also charged with insulting Saudi and Bahraini authorities, and for spreading false news that undermined Kuwait’s image abroad.
The charges were based on a number of tweets on his Twitter account, but Naqi told the interrogators that his account had been hacked, the lawyer said.