Gwent Police said the man was arrested in Hamburg on 12 August and will be extradited to the UK.
The man is believed to be Thomas Doran, who is wanted as part of Gwent Police's Operation Imperial - an investigation into serious offences against vulnerable adults on the Gwent Levels.
The extradition process can take six weeks, police added.
A 60-year-old man from Cardiff has been arrested in Germany on suspicion of conspiracy to kidnap.