The laminar flame speed in mixtures of partially dissociated NH3 in air under ambient temperature and pressure was studied via experimentation and kinetic modelling. The experiments were carried out using a vertical tube flame propagation method for low degrees of NH3 dissociation and a Bunsen burner method equipped with Schlieren imaging technique for high degrees of NH3 dissociation. Chemkin modelling incorporating three different reaction mechanisms, namely, the Okafor, Otomo, and Mathieu and Petersen mechanisms, respectively, was undertaken to predict the laminar flame speed. The degree of NH3 dissociation varied from 0 to 50% (0–37.5 %v/v H2 in the fuel mixture with a fixed H2/N2 ratio of 3) with equivalence ratios of 0.9–1.2. The experiments showed that a higher degree of NH3 dissociation led to a larger initial flame kernel size upon ignition and brighter flames. Regardless of the equivalence ratio, the laminar flame speed increased with increasing the degree of NH3 dissociation, from 0 to 50%, the maximum laminar flame speed increased from 7.8 to 22.7 cm·s−1 at the equivalence ratio of 1.10. The Okafor mechanism offered the best predictions of the experimental data while the Otomo and Mathieu and Petersen mechanisms over-predicted the laminar flame speed. The production of key radicals including OH, H, O, and NH2 was enhanced in the presence of H2 and, thus the conversion of NH2 to NH, HNO, and NNH, leading to a significant increase in the laminar flame speed.