As a member of the parliament's upper chamber, the House of Lords, Alli is required to register his interests in line with a parliamentary code of conduct.
The investigation, announced on the parliament's website on Wednesday, said Alli might have potentially breached certain sections of the code of conduct.
He is a major Labour donor.
Starmer and some of his top ministers came under fire last month for accepting donations from Alli to pay for clothing, for hosting parties and foreign travel.
The controversy forced Starmer to say he would not accept any more donations to pay for his clothing.
Deputy Prime Minister Angela Rayner and Finance Minister Rachel Reeves will also no longer accept them.
Alli, a media tycoon known in political circles for years, was brought to wider public attention in recent weeks due to scrutiny of his donations to Starmer.
Starmer has argued Lord Alli was motivated to help financially because he wanted Labour to win the election.
According to an update published on the UK parliament's website on Wednesday, Alli is being investigated by the Lords' commissioner for "alleged non-registration of interests leading to potential breaches of paragraphs 14(a) and 17 of the 13th edition of the code of conduct".
These rules relate to making clear what the interests are that might be reasonably thought to influence a member's parliamentary actions and ensuring entries are up to date.
with PA
