The Copenhagen Convention,^[1] which came into force on 14 March 1857,^[2] is a maritime treaty governing transit passage through the Danish straits.

It has abolished the Sound Dues and all Danish straits have been made international waterways free to all commercial shipping. Its provisions were later reaffirmed by Article 282 of the Treaty of Versailles, becoming binding for its parties. Although the Convention does not cover warships, military vessels have also been free to traverse the straits, regulated by the general international rule of innocent passage through international straits and the Royal Ordinance of 1976.^[3]

It had been increasingly evident that the Sound Dues had a negative impact on the port and merchants of Copenhagen, although the dues delivered by then one eighth of the Danish state income; and the world's seafaring nations were becoming less and less tolerant of these tolls and restrictions. In compensation for the abolition, the Danish state received a one-time fee of 33.5 million Danish rigsdalers,^[4] paid to Denmark by the other European shipping nations who signed the convention. Of the total fee, Great Britain paid approximately one third, and Russia another third.^[5] A similar convention between Denmark and the United States, signed in Washington the same year, gave American ships free passage in perpetuity for a one-time fee of $393,000.^[5]

A number of canals were built in part due to the trouble to pass the Danish straits.

• Kiel Canal
• Göta Canal
• Eider Canal