The Ninth-of-May (1948) Constitution was a constitution of Czechoslovakia in force from 1948 to 1960. It came into force on June 9, shortly after the communist seizure of power in the country on 25 February 1948. It replaced the 1920 Constitution. President Eduard Benes refused to sign it and resigned. This (not yet fully Communist) constitution was openly violated by the KSC, the government and many individuals throughout the period of its being in force, especially regarding the provisions on private ownership and human rights.

The constitution had been prepared since the summer 1946. There have been many disputes concerning the nationalization, the relation of the Czechs and Slovaks and other crucial issues, but after the Communist take-over in February 1948, the Communist concept was largely applied. Although the constitution did not establish the vanguard role of the KSC within the Czechoslovak state and government administration under the Leninist principle of democratic centralism (these were provisions only of the following "socialist" 1960 Constitution of Czechoslovakia), it confirmed the KSC had in Czechoslovakia the absolute power, just as other Communist parties in Communist countries.

The constitution declared that the economy of Czechoslovakia is based on nationalized industries, nationalized trade and a nationalized financial sector. The government sector was declared the basis of the economy, but it protected the private sector and cooperatives too. It also granted a small degree of autonomy to Slovakia, which was given its own legislative body and governmental structure, although these were made subordinate to the central authorities in Prague. The parliament remained to be called National Assembly.