The move could help President Recep Tayyip Erdogan to centralise powers in a powerful presidency, which until he took office in August was largely a ceremonial role in Turkey.
Fourteen members of the court voted against decreasing the threshold while two members voted in favour, Anatolia news agency reported.
The court ruled it was not "competent to change the threshold," it added.
Turkey is set to hold legislative elections in June, with the ruling Justice and Development Party (AKP) co-founded by Erdogan aiming for a thumping majority to change the constitution and boost the president's powers.
The vote of parties which fail to pass the threshold are re-distributed proportionally, which means the AKP would be the prime beneficiary.
The decision has dealt a blow to Turkey's main pro-Kurdish party People's Democratic Party (HDP), whose support level is just below the threshold. The party has long fielded its candidates as independents to go round the barrier.
