Prior to Ranieri's appointment in July, work was handled by Asus UK's in-house comms team.
The agency has been tasked with handling PR for Asus' systems division, which includes laptops, tablets, smartphones, desktop PCs and wearables, and will focus on how the devices were designed.
The brief also includes promoting Asus as an "innovative brand" within publications such as PRWeek sister title Stuff, and raising awareness among consumers, the agency said.
Asus UK PR manager Adam King said: "Ranieri has a reputation for clear-cut and front-of-foot communications with the tech media that is hard to beat. The team has a deep industry knowledge that, tied in with its extensive media network, is exactly what Asus needs to underscore its position in the market."
Ranieri, which was founded in 2002 by Pietro Ranieri, has 17 staff and grew revenue by a third to £1.2m last year. Its clients include audio products businesses JBL and AfterShokz.