Abstract: Using air with femtosecond laser beams to generate pulsed THz waves through third-order nonlinear optical processes has already been demonstrated. As its reciprocal process, in theory, it should be possible to use air as the sensor to detect pulsed THz waves. We report the first demonstration of time-resolved detection of broadband THz waves by measuring the THz field-induced second-harmonic signal in ambient air and/or ionized air (laser-induced air plasma). Experimental results and a theoretical analysis of incoherent and coherent detection are presented. A spectroscopic bandwidth of over 8 THz was observed for the air-breakdown coherent-detection system. The highest dynamic range demonstrated with an air sensor was 30 dB.