Swinson beat Ed Davey in the race to succeed Vince Cable as the leader of the Liberal Democrats in July.
Rathe revealed the move on Twitter, ending a three-year stint away from politics.
...and just to save you all a little bit of time:— Ben Rathe (@benrathe) August 3, 2019
1) Yes, I did once accidentally organise a visit to a dogging site.
2) Yes, I am also ‘officially’ the UK’s biggest thrill seeker.
3) No, you can’t have the Carly Rae Jepsen video.
He told PRWeek: "I’m delighted to be going back into politics at such an interesting time. I am excited to get started."
During his break from politics, Rathe served as the senior communications manager for The Investment Association; senior press and PR officer for the UK human resources trade association, the CIPD; and public affairs and digital media manager for the Consumer Finance Association.
He carried out several roles for Nick Clegg between 2012 and 2015, including policy and communications officer, senior press officer, events manager and constituency communications manager.
He was also a parliamentary adviser to the Liberal Democrats for the second half of 2015.
Swinson is not the only new leader shaping a new team of aides. Boris Johnson has also brought back some familiar faces to Number 10 after becoming UK prime minister.