The deal with Nycomed will allow Japan's largest drugmaker by revenue to tap into emerging markets demand at a time when pharmaceutical firms worldwide are battling revenue fallout from patent expirations.
It is the latest in a series of acquisitions for Japanese firms as they look to make the most of the strong yen and boost their presence in more lucrative markets overseas, offsetting limited domestic opportunities.
Nycomed has strong sales in emerging markets and Takeda has agreed to purchase the shares from private equity funds as it seeks to boost its presence in these markets. The deal excludes Nycomed's dermatology business.
Among Nycomed's products are treatments for lung disease, which Takeda said it expects to be a major source of revenue growth.
"Nycomed enables Takeda to maximise the value of our portfolio and gives us an immediate strong presence in the high-growth emerging markets while doubling Takeda's European sales," Takeda President Yasuchika Hasegawa said in a statement.
Takeda said it will finance part of the transaction through a loan of about 600-700 billion yen ($7.4-8.5 billion) and plans to complete the acquisition by the end of September, subject to antitrust clearance.
But ratings agency Moody's said it had put Takeda's Aa1 rating on review for a possible downgrade.
It warned that "the transaction would weaken Takeda's excellent balance-sheet liquidity and balance-sheet structure. Both are currently the best for any rated pharmaceuticals company worldwide."
Best-known for its Actos diabetes treatment, Takeda generates nearly half of its revenue overseas, but Asia and other regions outside of North America and Europe made up only two percent of its fiscal 2010 sales of 1.42 trillion yen.
The Swiss firm has some 12,500 employees worldwide and has subsidiaries in more than 70 countries, with a strong presence in Europe and in fast growing markets such as Russia, Latin America, Asia and the Middle East.
In 2010, Nycomed's turnover totalled $3.2 billion, ranking 28th among global pharmaceutical companies.
It specialises in gastroenterology medicine as well as treatments against respiratory and inflammatory diseases.
Takeda shares closed 0.53 percent higher at 3,790 yen in Tokyo trade on Thursday.
The deal continues a recent Japanese spending spree in the pharmaceutical space. In March Japanese drug maker Daiichi Sankyo said it would spend up to $935 million to buy California-based Plexxikon Inc.
Earlier in February pharmaceutical company Kyowa Hakko Kirin, a subsidiary of drinks giant Kirin Holdings, said it will buy Britain's ProStrakan Group PLC in a deal worth £292 million ($472 million).
And Astellas Pharma said it had agreed to pay around $1.3 billion to US drug developer Aveo Pharmaceuticals to jointly develop an experimental cancer treatment.
Astellas last year bought US firm OSI Pharmaceuticals for $4 billion.