The company has 70 offices around the world and provides consultancy services on infrastructure projects including London's Crossrail project.
Neil Innes, Halcrow's management information services architecture and planning manager, said outsourcing web filtering would take associated hardware and software off-site as well as enable junior staff to handle any system changes using a web interface.
Innes said the rising use of the web and malware attacks had made content filtering a priority for Halcrow.
"Creating a wider security perimeter using an 'in the cloud' service prevents malware from getting anywhere near our network, which is an important security concept," he said.
Innes said the management capabilities enabled by the service had virtually eliminated the need to investigate malware incidents.
"Our security manager was previously dealing with up to 50 investigations a year taking two days each, so this is potentially saving us 100 days a year," he said.
Halcrow chose to implement the web filtering system as a service to avoid the cost of deploying hardware and software to every site.
Innes said the company had chosen software-as-a-service provider ScanSafe because of its compatibility with Halcrow's existing IT architecture.