Controlled by the family of former Prime Minister Silvio Berlusconi, Mediaset now holds 15.1% of ProSieben after it bought a 5.5% stake in the group on Monday through its Spanish unit Mediaset Espana.
No decision has been taken yet, but the Italian group could as a next step raise its stake in the German company to just below 20%, the source said.
Established European broadcasters are looking for ways to respond to growing competition from streaming services such as Netflix Inc and online advertising giants like Alphabet Inc and Facebook Inc.
Mediaset has invested around 520 million euros so far and has become ProSieben's largest investor.
In response to Mediaset's Monday stake increase, ProSieben said it welcomed such investments as an affirmation of the group's strategy.
ProSieben Chief Executive Max Conze is seeking to move away from commercial TV towards the more profitable businesses of digital, content production and e-commerce.
The Munich-based broadcaster is. however. sceptical about the merits of cross-border European TV mergers, showing a preference for partnerships such as a joint venture in advertising technology with competitor RTL Group.
A second source said on Tuesday that Mediaset was not planning to increase its stake at this stage but did not rule it out in the future.
The Italian broadcaster is not planning a takeover of the German company but wants to convince ProSieben that an agreement over a closer tie-up is necessary, one of the sources said.
Mediaset aims to press ahead with a corporate overhaul and merge its Italian and Spanish business into a Dutch holding company, dubbed MediaforEurope, to pursue pan-European tie-ups, including a potential merger with ProSiebenSat.1.
That would allow it to provide streaming services across Europe.
Vivendi, Mediaset's second largest shareholder, is challenging the plan in the courts claiming it includes a governance structure that would favour Berlusconi's holding Fininvest and tighten its grip over the broadcaster.
Mediaset shares were up 3.2% at 1030 GMT, while ProSieben was up 0.9%.
By Elvira Pollina