The Vidar and Poseidon wind facilities will be developed at a site located between 25km and 27km from the Swedish west coast, north of Gothenburg.
With a combined capacity of 2.8GW, the two facilities will primarily consist of floating wind turbines that can generate 10.5TWh of clean energy a year.
Vattenfall and Zephyr plan to develop early-stage projects and apply for the necessary permits for establishment together.
The financial details of the deal have not been disclosed.
Vattenfall Wind Business Area head Helene Biström said: “Vattenfall aims to enable fossil-free living and to support industries and society at large on the Swedish west coast by providing more renewable electricity and this is an important step towards that target.
“These two projects will add significant volume of cost-effective fossil free electricity in an area with high demand.
“They will also support our development of the important floating technology that is expected to reduce the environmental impact of the wind farms, as well as enable offshore energy production at sites further away from shore.”
Zephyr Renewable CEO Martin Pettersen said: “Zephyr is delighted to be able to continue with the development of the Poseidon and Vidar projects together with Vattenfall.
“There is a pressing need to ensure that new electricity generation can become a reality in southern Sweden, both for the climate and our society, which is facing a sharp increase in electricity consumption.
“Poseidon and Vidar, as large-scale offshore wind power projects, are in a good position to contribute significantly towards this.”
Vattenfall has around 12 active offshore wind farms in Europe and 9GW of capacity in development.
Last month, the company secured permission for the 640MW Kriegers Flak offshore wind farm from the Swedish government.