Spiral: The Bonds of Reasoning (Japanese: スパイラル〜推理の絆, Hepburn: Supairaru: Suiri no Kizuna) is a Japanese manga series written by Kyou Shirodaira and illustrated by Eita Mizuno. It was published by Enix and then Square Enix in Monthly Shōnen Gangan from 1999 to 2005 and collected in 15 bound volumes. The series focuses on Ayumu Narumi and his efforts to solve the mystery of the Blade Children, cursed geniuses with cat-like eyes lacking a seventh rib bone. A prequel series by Shirodaira and Mizuno, called Spiral: Alive, was published in Gangan Wing and Monthly Shōnen Gangan from April 2004 to June 2008 and collected in five bound volumes.

Spiral: The Bonds of Reasoning was originally licensed for an English language release in North America by Tokyopop, however the company dropped the series. Yen Press acquired the license for Northern American release, and published the first volume in October 2007.

The series was adapted as a twenty-five episode anime television series broadcast on TV Tokyo from October 1, 2002 until March 25, 2003. The anime is licensed in Region 1 by Funimation, who released it on DVD and broadcast it on the Funimation Channel, along with the programming block on Colours TV in 2006. The series was also adapted as four light novels and a series of CD dramas.