Snam said the De Nora stake deal, which it expects to complete in the first quarter, will strengthen its position in green hydrogen technologies.
Last year Snam, which makes most of its revenue from gas transport in Italy, set up a hydrogen unit as part of its plans to position for the transition to cleaner energy with a pledge to spend more on new green business lines.
"The partnership with De Nora will allow us to be a leader in the development of green hydrogen, which is becoming a key feature of the decarbonisation agenda," Snam CEO Marco Alvera said in a statement.
Founded in 1923 in Milan, De Nora is a leading manufacturer of electrodes that can be used in the production of so-called green hydrogen production, which is produced from renewable energy sources using devices called electrolysers.
The electrode business accounts for 60% of De Nora's revenue, which total more than 500 million euros.
De Nora, which also holds 34% of a joint venture with Germany's ThyssenKrupp involved in several large global hydrogen projects, is expected to post a 2020 core profit of 90 million euros.
Snam said De Nora, which also operates in the water treatment sector, was ideally positioned to be a global technology-based listed company.
It said the asset would fit into its plans to set up a new investment platform next year to meet growing interest in clean energy and decarbonisation.
The platform will grow Snam's exposure to companies and projects relevant to decarbonisation, it said.
Blackstone invested in De Nora in 2017 and expanded the company in China. De Nora's manufacturing plants are spread across 12 sites in Germany, Italy, the United States, Brazil, Japan, China and India.
The U.S. investment firm, which held the stake through funds managed by Blackstone Tactical Opportunities, was advised by Lazard while Snam was advised by BofA Merrill Lynch.
($1 = 0.8445 euros)
(Reporting by Stephen Jewkes; Editing by Agnieszka Flak and Alexander Smith)