The Corrupt Practices Investigation Bureau said it was interviewing the minister, S Iswaran, who was "assisting" in a case, but did not provide further details.
Prime Minister Lee Hsien Loong on Wednesday said he had "instructed Minister Iswaran to take leave of absence until these investigations are completed".
The corruption bureau began investigations on July 11, he added in a statement released by his office.
Iswaran did not immediately respond to Reuters' request for comment. The 61-year-old joined Lee's cabinet as a junior minister in 2006 and held the trade and communications portfolios before becoming transport minister in May 2021.
Graft investigations involving ministers are rare in Singapore, where civil servants are paid handsomely to discourage corruption. Many cabinet ministers' annual salaries exceed S$1 million (nearly $750,000).
In May, Lee ordered an investigation into the circumstances around two heavyweight ministers' rental of state-owned homes. They were cleared of wrongdoing in June.
($1 = 1.3384 Singapore dollars)
(Editing by Kanupriya Kapoor)
By Chen Lin
