Developer of the 30MW Windfloat Pacific project, off the coast of Oregon, northwest US, said the two companies had been long-time partners.
Principle said MI&T had "played a key role" in developing the semi-submersible floating technology used in the WindFloat foundation and the merger will allow the company to "focus resources" on its key projects.
A prototype of the WindFloat foundation, fitted with a Vestas V80 2MW, turbine has been operating off the coast of Portugal since 2012.
The full-scale 30MW project in the Pacific Ocean is set to use five Siemens 6MW turbines.
In May 2014, Principle Power was awarded $47 million by the US government's Department of Energy to fund the construction of the project.
The group has also announced the appointment of Joao Metelo as its new CEO, having previously been COO and acting CFO at Principle.
Metelo has 15 years in the energy sector - he has also been CFO at EDP Renewables North America.