It has been proposed that activity in human primary visual cortex (V1) is necessary but not sufficient for conscious awareness, but direct physiological evidence for the activation of human V1 in the absence of awareness has been elusive. Here, we used functional MRI to measure activity in V1 while participants viewed masked and invisible orientated gratings. Remarkably, when participants were unable to report the orientation of a masked grating they were viewing, we could nevertheless predict its orientation significantly better than chance, based only on a single brief measurement of activity from their primary visual cortex. This was achieved by using multivariate pattern-recognition to substantially improve the ability to detect orientation signals in V1 (see also Kamitani & Tong, VSS 2004). Thus, activity in primary visual cortex conveys orientation-specific information that is inaccessible to conscious report. Orientation can therefore be represented unconsciously in V1, suggesting that information must be relayed to another region of the brain to be represented in conscious experience.