This communication reports a DNA tetrahedron whose self-assembly is triggered by an acidic environment. The key element is the formation/dissociation of a short, cytosine (C)-containing, DNA triplex. As the solution pH value oscillates between 5.0 and 8.0, the DNA triplex will form and dissociate that, in turn, leads to assembly or disassembly of the DNA tetrahedron, which has been demonstrated by native polyacrylamide gel electrophoresis (PAGE). We believe that such environment-responsive behavior will be important for potential applications of DNA nanocages such as on-demand drug release.