The EU's top court says it is wrong for Germany to impose a basic language requirement on Turks who want to join their spouses in Germany.
The German language test makes family reunification more difficult and is not compatible with an EU-Turkey agreement reached in 1970, the judges said.
Germany brought in the rule to boost integration and stop forced marriages.
About three million ethnic Turks live in Germany, and half of them are German citizens. Turkey is not yet in the EU.
The European Court of Justice (ECJ) ruling is specific to Turkish migrants to the EU.
However, it could influence court cases affecting similar language requirements in the EU. ECJ rulings are binding EU-wide.
Non-EU nationals applying to stay permanently in the UK have to demonstrate intermediate English language skills and pass a "Life in the UK" test.
EU nationals enjoy visa-free travel throughout the 28-nation EU and the residence rules are less strict for them.
Germany's ruling Christian Democrats (CDU) said the ECJ ruling only applied to Turks, and the language test would still be a requirement for immigrants' spouses from other countries.
Right to family life
The ruling concerns Naime Dogan, a Turkish national wishing to join her husband - also Turkish - who has lived in Germany since 1998.
In January 2012 the Germany embassy in Ankara refused to grant her a visa for family reunification purposes, on the grounds that she was not proficient in German.
The judges said the German language requirement failed to take account of individual circumstances. They argued that the right to family life outweighed the justification for the measure.
A centre-left Social Democrat (SPD) politician in Germany's ruling coalition, Aydan Ozoguz, welcomed the ruling. She said dropping the test would not make it harder to integrate immigrants. Existing German language courses in Germany were well designed to help immigrants integrate, she said.
Talks on Turkey's EU membership bid reopened last November, but Ankara is still a long way from meeting all the stringent EU criteria.
Since they began in 2005 the talks have stalled over issues such as freedom of speech and democracy, treatment of religious minorities, judicial reform and ongoing tensions with Cyprus, an existing EU member.