The 109 million euro ($116 million) stakebuilding strengthens the grip of Italian investors on a company in which France's Amundi - Europe's biggest asset manager - has also taken a stake.
Amundi has a strong presence in Italy having spent 3.6 billion euros in 2017 to buy peer Pioneer from UniCredit.
Formerly backed by the state and focused on promoting national champions, private equity fund Fondo Strategico Italiano (FSI) targeted a stake of up to 9%. It secured just above the 7% minimum deal size with a 7.5% premium versus Tuesday's closing price.
Shares in Anima gained as much as 7% to trade at 4.33 euros, just shy of the 4.35 euros offered by Mediobanca on behalf of FSI, before paring gains.
A person close to the matter said FSI was beefing up its stake, with volumes traded totalling nearly 40 times the past month's daily average by 1547 GMT. FSI declined to comment.
Anima shares hit a 29-month low in September at 2.8 euros, but have rallied since.
FSI's investment comes a week before a deadline to file slates of candidates to Anima's new board. Based on its by-laws, a 9% stake ensures its owner a board seat, though a 7% stake should also be enough for this aim.
Anima's biggest investor is Italian lender Banco BPM with a 20.6% stake, followed by the national post office Poste Italiane with 11%. Both are set to appoint their own representatives on the board.
Anima has a commercial partnership in place with both Banco BPM and Monte dei Paschi di Siena, the state-owned bank which Rome must re-privatise and of which Banco is seen as a potential buyer.
"This move could be a bet on the possible reshuffle of the Italian banking system with the creation of a bigger banking group, which could control Anima at the end of the story," broker Akros said.
Amundi became the third-largest investor in Anima last year with a 5.2% stake, in a move that focused attention on French interest in the Italian financial sector.
Amundi acquired its Anima stake a month after Amundi's owner, French bank Credit Agricole, bought 9.2% of Banco BPM, becoming its single largest investor.
Anima turned to the government of then Prime Minister Mario Draghi where officials shared its concerns about the importance of keeping the asset manager in Italian hands, sources told Reuters at the time.
Of Anima's 177 billion euros of assets under management, some 100 billion euros are invested in Italian government bonds.
FSI has not disclosed the reasons behind its decision.
FSI was an investment vehicle of CDP until the Italian state lender last year sold its 39% stake to FSI's partners, including CEO and former Merrill Lynch banker Maurizio Tamagnini.
CDP is now one of the investors in funds managed by FSI, which has raised 1.4 billion euros, of which 600 million euros have been invested in financial service and technology businesses. ($1 = 0.9340 euros)
(Reporting by Valentina Za; Editing by Keith Weir and David Holmes)
By Valentina Za