Knowledge of the complete stiffness tensor of a composite material is a prerequisite for the simulation of mechanical behavior of components. This tensor consists of nine (for an orthotropic composite) respectively five (for a transverse isotropic composite) independent elastic constants. Usually these constants are determined by measuring stress-strain relationships in a set of mechanical tests followed by an inverse fitting procedure. These require the application of different loads (tensile, shear) in different directions – this is only possible using specimens with different geometry and orientation. This contribution describes a non-destructive ultrasonic time-of-flight measurement which facilitates the determination of the stiffness matrix of a composite using a single plate shaped specimen. Unlike in mechanical tests, there is no load applied to the specimens, i.e. a single specimen will yield consistent results in repeated measurements. This makes the presented method particularly suitable for monitoring property changes (e.g. due to ageing or fatigue). For the time-of-flight measurement, the composite plate is immersed in a fluid and subjected to an ultrasonic pulse under variable angles of incidence and the pulse is detected by a receiver after passing through the plate. The propagation velocity of the longitudinal and shear ultrasonic waves travelling through the specimen is then calculated for each angle of incidence from the time of flight of the acoustic pulses. The acquired phase velocities were used to fit a complete set of elastic constants in a ceramic matrix composite with tetragonal symmetry. The elastic constants compared favorably with results obtained by laser vibrometry. While the elastic constants in the in plane direction were of high precision for both methods, out-of-plane constants were more consistent using the time-of-flight method.