Ukrainian-born Alexander Temerko is believed to have told friends he thinks the former Government Chief Whip is best suited to take the reigns of responsibility.
The former oil company boss, who has given more than a million to the Tories in the past, believes Mr Williamson has proven his ability during his short time in the Defence brief.
Writing on a Conservative friendly website Mr Temerko has commended the South Staffordshire MP for his role.
He said: “After years of defence cuts, with even more to come according to recent reports, there is a legitimate need to question the ability of the Armed Forces to keep Britain safe in this dangerous era.
“Are we prepared?”
Praising the politician for his commitment to increasing funding for the Armed Force he even compared Mr Williamson to Churchill.
He said: “Just as Churchill recognised before the second world war, Gavin Williamson knows the UK needs to increase spending.”
Mr Temerko has already lost his patience with Theresa May and last year called on the Conservative Party to elect a new leader.
At the time he said: “We must start the process of finding a new leader now and it is important to elect this leader by way of a democratic vote with no coronation or another tricky method.”
The donor’s support for the Defence Secretary will come as a blow to the leadership ambitions of Boris Johnson.
The top party man was previously believed to have given his backing to the former London Mayor.
Mr Temerko said: “I think Gavin is a very loyal and most effective member of Mrs May’s team and the Cabinet.
“He is, as any top politician, a really ambitious man, but I do not know about his leadership intentions today.”
Gavin Williamson became Defence Secretary in November last year after Michael Fallon resigned from the post following allegations about his personal life.
Since getting his brief the South Staffordshire MP has been vocal in his calls for the need of more funding for the army and has not been afraid to publicly call for Philip Hammond and the Treasury to give more money to his department.
Despite being a Remain campaigner during the EU referendum, the minister has won plaudits from Brexiteers after he reportedly told his department to be more patriotic.
The Defence Secretary was said to have been shocked to see EU flags in the Defence Ministry building, including outside important meeting rooms.
In a bid to quickly put right the out-of-date decor, Mr Williamson ordered the flags be replaced with Union Jacks.
One source is believed to have said: “Gavin said he saw no reason why they should waste any time on delaying the removal of the EU flags.
“He wants more British pride displayed around the building as Britain gets ready to quit the EU.”