Kathryn Stone is investigating the Shadow Foreign Secretary under four different sections of the rules, including areas covering earnings, gifts and foreign travel.
The investigation, opened on Wednesday, was included in an update to the Parliament website.
Ms Stone was looking at possible breaches to the MPs’ code of conduct under areas covering earnings, gifts and foreign travel.
The MP for Tottenham in London joins Labour leader Sir Keir Starmer in being investigated by the watchdog.
Sir Keir insisted he was “absolutely confident” he had not broken the rules, as he blamed the investigation on “administrative errors” in his office.
Under the rules, MPs must register changes to their financial interests within 28 days.
Mr Lammy’s financial register contains a series of interests registered after that period, including a speech in the US on the invasion of Ukraine.
And a sum of £3,280 received from the Canary Wharf Group on December 1 for a speech and question-and-answer session was not registered until May 27.
David Lammy
Aaron Chown
Labour leader Sir Keir Starmer is also under investigation
House of Commons