Max Martin co-wrote “Actually Romantic” with Taylor Swift and Shellback for The Life of a Showgirl. Martin also co-produced the song and contributed keyboards and recording work.
The collaboration continued Swift’s long creative relationship with Martin and Shellback, who had previously worked with her on major songs from Red, 1989, and Reputation. “Actually Romantic” reflects that partnership through a concise pop-rock arrangement with alternative-rock and power-pop elements.
Max Martin is sometimes confused with the album’s other Swedish producer, Shellback, whose professional name belongs to Karl Schuster. Both are credited on the song, but the question asks specifically for the additional co-writer besides Swift and Shellback. Songwriting and production credits overlap here, with all three listed in both roles.