The court said there was not enough evidence to try the case.
Saudi Arabia has long suspected official involvement in the 1990 disappearance of businessman Mohammad al-Ruwaili and the murders of four members of Saudi Arabia's diplomatic staff in Bangkok. Its government has suggested that the slayings were linked to the theft of $2 million worth of royal jewels from a palace in Riyadh by a Thai cleaner, many of which were never recovered.
Saudi Arabia downgraded relations with Thailand following the crimes and ties have never fully been restored.
Monday's ruling cleared charges against former police Lt Gen Somkid Boonthanom and four other officers. All had been facing charges of premeditated murder and the illegal detention of al-Ruwaili.
In 2010, the Thai police planned to promote Pol Lt Gen Somkid to be an assistant national police chief, prompting strong criticism from Saudi Arabia, which said the move could jeopardise efforts to restore normal diplomatic relations. Pol Lt Gen Somkid later declined to accept the promotion.
In 1989, $2 million worth of gems and jewellery were stolen from a Saudi Arabian prince's palace in Riyadh. A Thai worker at the palace was subsequently arrested and jailed for the theft after returning to Thailand. A portion of the gems were recovered and returned by Thai police. But some turned out to be fake, leading to suspicions that senior police and members of Thailand's power elite kept the loot and ordered a cover-up.
Ateeq al-Ruwaili, the businessman's elder brother who is in Bangkok for the ruling, was due to hold a press conference later on Monday.