An update to the site on Wednesday said the commissioner, who is responsible for monitoring the operation of the House of Commons code of conduct and registers of interests, had opened an investigation on April 29.
It listed the matter under investigation as "registration of interest".
MPs are expected to provide information about any financial interest or benefit they receive which others might reasonably consider to influence their actions or words.
No further details of the allegations against Reeves were provided, in line with the commissioner's usual practice.Â
The fact that an inquiry is taking place does not mean that rules have been broken, the website stated.
The finance ministry did not immediately respond to a request for comment.
If the complaint against Reeves is upheld, the commissioner could ask her to acknowledge and apologise for the breach or, in more serious cases, refer it to a separate committee which can recommend sanctions.
Serious sanctions, such as suspension, must be approved by the House of Commons - parliament's lower chamber.
There are currently three other MPs being investigated over their registration of interests.
