The decision will be a blow to a government that has seen three ministers quit in recent weeks, while it battles to jumpstart an underperforming economy and ensure it has the funds to deliver on a delayed election promise of cash handouts for 50 million people.
The government did not immediately respond to a request for comment.
Pichit has long been a close aide of the billionaire Shinawatra family, whose parties have won all but one election in the past two decades.
As a lawyer he fought, and lost, big court cases against former premiers Yingluck Shinawatra and brother Thaksin Shinawatra, the founder of Srettha’s ruling Pheu Thai party.
Pichit was jailed for six months in 2008 for contempt of court after an alleged attempt to bribe court officials with 2 million baht (US$55, 218) hidden in a paper grocery bag. His law license was suspended for five years after the incident.
The 40 senators said Srettha violated the Constitution by appointing Pichit, who lacks integrity and ethical standards required by the Constitution to hold a ministerial position.
The government has said it had carefully vetted Pichit’s qualifications and was confident it could defend his appointment in court.
Its critics say Pichit got the job due to his close relationship with Thaksin, who has been a towering figure in Thailand for more than two decades.
Thaksin still holds significant sway in politics, despite officially being retired and having spent 15 years in self-imposed exile until his return last year.
The government said Pichit had been picked because of his suitability for the ministerial role in the premier’s office.
