The company added that the tunnel will help reassure passengers that FNA is a coronavirus (COVID-19) secure environment and boost traffic through the airport.
Westminster provides security and screening services for the airport and said it had promised earlier in the year to provide it with a mass sanitisation tunnel.
As part of its Corporate Social Responsibility (CSR) programme, the group's Westminster Foundation provides support and assistance to communities where the company operates.
Moses Tiffa Baio, the Director-General of Sierra Leone Civil Aviation, described the provision of the sanitisation tunnel as 'A promise made, a promise delivered'.
FNA has seen a steady increase in passenger numbers since international flights were resumed in July this year, Westminster said, and in the last few days confirmation has been received that further airlines are recommencing operations in December.
Current airlines are also increasing their flight schedules in line with previous operational levels.
In a statement, Joanna Fowler, Westminster’s Head of Managed Services and Trustee of the Westminster Foundation, said: "The COVID 19 pandemic has had a catastrophic effect on the aviation industry and Westminster is at the forefront in the fight to mitigate the impact on business and travel worldwide.
"We are pleased to be able to assist the authorities in improving the safety, as well as the security, of the airport and are equally pleased to see passengers and airlines returning."