The key role will see her serve as the 'eyes and ears' of the new PM in Parliament, with a remit to support her on a day-to-day basis, liaise with MPs and help administer Prime Minister's Questions.
Ms Webb previously worked with the PM when she was International Trade Secretary and backed her during the Conservative leadership campaign.
She said: "It is a huge honour to be asked to do this job for the new Prime Minister and I will do all I can to support her and her government as it sets about tackling the challenges the country faces.
"I have been a big supporter of Liz since I worked with her when she was International Trade Secretary. I supported her leadership campaign because I passionately believe in her policies of putting more money into people’s pockets through tax cuts.
"I passionately support her ideas to help our country through the cost-of-living crisis, her priority of getting the economy moving and increasing investment and her steadfast support of Ukraine.
"Along with improving the NHS and safeguarding our energy security we have much to do and I am looking forward to playing my part in the years to come.
"I believe my new role will do much to help the West Midlands too and my focus remains firmly on Stourbridge and my priorities of jobs, jobs, jobs, protecting our greenbelt and levelling up our communities."
The unpaid role is often considered a stepping stone to ministerial positions.
Ms Webb, who was elected in 2017, has previously been a PPS at the Department for International Trade and for the Defence Secretary, Ben Wallace. She briefly served as an assistant whip in Boris Johnson's final administration.
Another ally of Ms Truss, Aldridge-Brownhills MP Wendy Morton, has been appointed Chief Whip, while Wolverhampton South West MP Stuart Anderson has been reappointed as an assistant whip.