The city of Hamburg had rejected Juergen Roemer's plea to be put in a more favourable tax category.
But that decision may amount to sex discrimination, the European Court of Justice (ECJ) ruled.
Mr Roemer claimed he was entitled to 302 euros (£265) a month more.
The judges said that under German law same-sex partnerships - called "registered life partnerships" - are now comparable to marriage.
Mr Roemer had worked for the Hamburg city authorities for 40 years. He retired in 1990 and in October 2001 told his former employer that he had established a registered life partnership with his male partner.
Mr Roemer had been living with his companion since 1969 and German same-sex partnerships were given legal status in February 2001.
While many European states recognise homosexual civil unions, only Spain, the Netherlands, Belgium, Portugal, Sweden, Norway and Iceland legally acknowledge same-sex marriage.
The ECJ ruling on Tuesday may give legal weight to similar claims in other EU countries.
The judges said individuals could claim against a local authority for the right to equal treatment under EU law, even if national law contained no provision relevant to their case.
The EU's top court has ruled that a retired German man in a same-sex civil partnership should enjoy the same tax status as a married man when his pension is calculated.