A contaminant sensor (1) for detecting magnetizable contaminants (7) present in a lubricant flow is disclosed. A permanent magnet (3) is arranged movably inside a sensor housing (2). A sensor element (6, 11, 13), e.g. in the form of a distance sensor (6) or a pressure sensor (11), is arranged to detect a displacement of the permanent magnet (3) inside the sensor housing (2), and an indicator is arranged to generate an alert signal when a displacement and/or a rate of change of displacement of the permanent magnet (3) inside the sensor housing (2) exceeds a predefined threshold value. The permanent magnet (3) is arranged to move inside the sensor housing (2) in response to magnetizable contaminants (7) collected on an outer surface of the sensor housing (2).