This study investigated the influence of tartaric acid (TA) on the early hydration of NaOH-activated ground-granulated blast-furnace slag (GGBS) paste. Early hydration process of the paste with or without TA was characterized by the hydration heat using an isothermal calorimeter. In addition, inductively coupled plasma spectroscopy, X-ray diffraction, Fourier transform IR spectroscopy and scanning electron microscopy analyses were used as auxiliary tests. The results reveal that TA has a significant retarding effect on the hydration of NaOH-activated GGBS paste, finally leading to the initial setting time increasing with increasing TA. The complexation reaction of Al rather than Ca is the main reason for the observed retardation effect. The Al-complexed carboxylate salts formed by the complexation reaction of Al inhibit the depolymerization of the Si–Al phase, which leads to extend the time for SiO 3 2− and Al3+ to reach the saturation point in pore solution. And it subsequently delays the crystallization and nucleation of the main hydration products. However, TA has a little effect on the hydration products of NaOH-activated GGBS paste. The application of TA maintains the variety of main crystal products and the constant structure of gel products. The gel product still remains C–(A)–S–H with a primarily Q2 structure. Therefore, TA is an effective retarder for the NaOH-activated GGBS system.