The German developer said the 1,633 tonne substation platform was connected to its jacket foundation using a floating crane.
It was designed and constructed by Danish firms Bladt industries, Semco Maritime and consultancy firm ISC.
The substation will now be connected to the inter-array cables, which were installed last month and then Dutch cable company Tennet's export cables once they have been put in place.
Nordsee Ost is powered by 48 Senvion (formally Repower) 6.15MW turbines that are located 35 kilometres from the island of Helgoland.
Turbines started being installed at the beginning of July and all construction is due to be completed by the end of 2014 ahead of a brief testing period before commissioning in spring 2015.