Femtosecond laser-induced plasma spectroscopy for in situ ammonia (NH3) measurements was demonstrated in NH3/N2 mixtures. When a femtosecond laser at 800 nm was focused at the flow field, the parent NH3 molecules would be photolyzed to generate electronics excited NH fragments, and then indirect measurements of NH3 could be realized by detecting the NH fluorescence (A3Π-X3Σ-) at 336 nm. A detection limit of 205 ppm was achieved. This work is the first attempt, to the best of our knowledge, for ammonia measurements with a femtosecond laser, and the results are useful for the development of ammonia diagnostics.