The 30-year-old has been at Schalke since 2013, and gives Watford another defensive option to consider.
The Baggies have been linked with a move for the Leeds left-back Charlie Taylor.
The former German international would be a canny addition for Leeds United if they earn promotion.
Sunderland are believed to be interested in the versatile German.
Left-back Dennis Aogo, who was being touted as a potential Anfield addition in January, will move permanently to Gelsenkirchen.
Will Schalke sign Liverpool target Dennis Aogo despite his knee injury? – Quite likely after what general manager Heldt said.
Liverpool target Dennis Aogo has suffered a cruel blow.
A former German international was killed as a terrorist in Syria last month, it has been revealed.
Liverpool have been linked with signing Dennis Aogo.
Dennis Aogo of Hamburg is a much better long term replacement for Patrice Evra at Manchester United than Leighton Baines.