The pop star and her current boyfriend, DJ Calvin Harris, organised a double date with Jonas and the model in London over the weekend (27-28Jun15).
The two couples and another Swift pal, Karlie Kloss, took a boat trip on the River Thames and Swift posted a shot of the quintet enjoying a day out online.
She added the caption: "Little known fact: Karlie is secretly an unofficial historian/London tour guide. Kind of."
Swift dated Jonas when they were both teenagers and turned their awkward break-up into lyrics for the songs Forever and Always and You're Not Sorry.
Jonas isn't the only ex who has inspired lyrics in Swift's songs - she has also written tracks about her romances and break-ups with John Mayer and Harry Styles, among others.