A sedition charge against Future Forward alleged that it was influenced by the Illuminati and was seeking to overthrow the monarchy.

The Constitutional Court dismissed the charge. A guilty verdict could have seen the party banned.

But it is still facing several cases that could result in its dissolution.

Future Forward was founded less than two years ago. It has appealed strongly to young voters and won the third largest share of seats in last year's general election.

But the party's campaign to reform the military has angered supporters of the government, which is still headed by generals who led a military coup in 2014.

What were the allegations?

The dismissed complaint accused Future Forward's founder and leader, Thanathorn Juangroongruangkit, and other executives of being "part of an anti-monarchist movement".

Thailand has a constitutional monarchy, but the royal family is highly revered by Thais and wields considerable power. The country's lese-majeste law forbids any criticism of the monarchy and carries a hefty prison sentence.

The party says it supports the monarchy and, on Tuesday, a judge ruled that it had not "not acted... to overthrow" it.

The complaint also alleged that Future Forward was influenced by the Illuminati, which conspiracy theorists believe is an elite secret society that seeks world domination. It said the party's symbol resembles an upside-down triangle that is often associated with the mythical group.

The complaint, which has been ridiculed by Future Forward, alleged that the Illuminati played a role in the fall of monarchies across Europe and was now seeking to do the same in Thailand.

Supporters of the party celebrated when the ruling was announced

Pannika Wanich, a spokeswoman for Future Forward, said the case was an attempt to eliminate the party. "This is a deliberate political harassment," she said.

"We believe those cases are politically motivated," Mr Thanathorn told reporters following the ruling.

A party leader viewed as dangerous

Analysis by Jonathan Head, South-East Asia Correspondent

There were cheers from supporters of Future Forward as the judges of the Constitutional Court read out their verdict.

Some even called for Prime Minister Prayuth Chan-ocha to resign.

The court has a track record of dissolving dissident political parties on seemingly trivial grounds, but the bizarre accusation this time - that Future Forward was allied to the Illuminati in a campaign against the monarchy - was judged too flimsy to warrant such punishment.

But the party and its executives still face more than 20 other legal cases that could yet result in its dissolution.

Its youthful and charismatic leader, Thanathorn Jungruangroonkrit, a wealthy businessman, is viewed as a dangerous subversive by many conservative Thais.

Earlier this month, he joined a nationwide run to protest against Mr Prayuth's continued hold on power after last year's election. And he promised to lead more protests if his party is dissolved.

Thailand has been buffeted by political instability for years, largely a battle between supporters of the military and former Prime Minister Thaksin Shinawatra.

The country's military has a history of intervening in politics. Mr Prayuth, a retired general, led the military coup in 2014 and was elected as the country's civilian prime minister in June.

Mr Thanathorn has emerged as one of the fiercest critics of the prime minister. Last year, he faced charges of sedition which were filed by an officer in the government.