The move follows complaints from campaigners about a "severe lack of critical distance" between the ECB's decision-making bodies and corporate bankers in the G30.
Europe's central bank is tasked with regulating the financial sector and supervising other central banks in the Eurozone.
Transparency groups say any perceived or actual conflicts of interests pose a "great risk" to the ECB's integrity.
At the centre of the complaint is the ECB's Italian President Mario Draghi, who is a full member of the G30. But other members of its governing bodies have also reportedly contributed to the group, including ECB supervisory board member Julie Simpson.
On Thursday, GUE/NGL group MEPs welcomed the probe by Strasbourg-based ombudsman Emily O'Reilly.
German GUE MEP, Fabio De Masi, who is a member of the European Parliament's economic and monetary affairs committee and its banking union working group, said, "I welcome the investigation launched by the ombudsman into ex-Goldman Sachs boy Mario Draghi and his ties with the financial industry.
"I made a similar complaint to that of Corporate Europe Observatory a while ago. As a reaction to the financial crisis, the European Central Bank's supervisory role was extended.
"Mario Draghi's membership in the G30, a club of bank governors that he is also supposed to supervise, is highly problematic."
De Masi has previously posed questions to the ECB on the matter.
Corporate Europe Observatory, a Brussels-based campaign group, said the inquiry comes after the ECB "repeatedly dismissed" its concerns, saying the bank had "done nothing to assess if staff's deepening involvement with the G30 is in line with the bank's ethics rules."
Corporate Europe Observatory's financial policy researcher Kenneth Haar added, "If the European Central Bank does not take ethics seriously, how can they be expected to properly enforce monetary policy rules in the Eurozone?
"Our research shows that high-level ECB staff are much too close to representatives of the banks they are meant to supervise, and that the info they give out at G30 meetings goes unchecked.
"It is important that the ombudsman is now also looking at this proximity between ECB bankers and corporate representatives in the group."
Haar added, "All EU institution need to ensure transparency and avoid conflicts of interest - that goes for the ECB as much as it does for the European Commission and the European Parliament. The ECB has become a much more powerful institution lately, and it seems the ethics procedures in the Bank do not match this development at all."