Saab paid 340 million kronor ($49.6 million) for the ThyssenKrupp Marine Systems subsidiary, which the Swedish company said would come from existing funds.
"The impact of the transaction on Saab's results for 2014 is not considered to be significant," it said in a statement.
ThyssenKrupp Marine Systems has around 850 employees and counts the navies of Sweden, Australia and Singapore among its clients.
"The acquisition strengthens Saab's position as a comprehensive supplier of naval military systems," Saab said.
The subsidiary designs, builds and maintains not only submarines but also surface vessels and submarine rescue vehicles.
Sweden is one of the world's biggest arms exporters per capita, and Saab is the country's main defence equipment company.
The business unit, renamed Saab Kockums, is expected to keep its operations in the southern Swedish ports of Malmö, Karlskrona and Muskö.
The acquisition "makes Saab one of few companies in the world with the ability to develop, produce and deliver comprehensive defence solutions for air, land and sea", chief executive Haakan Buskhe said when the company published its second-quarter results last week.
The negotiations between the companies were first announced in April, when they signed a memorandum of understanding.