During the optimisation of the synthesis conditions of non-hydrated potassium dawsonite, a compound which has been recently shown to be involved in the alkaline hydrolysis of calcium aluminate cement (CAC), a new closely related hydrated phase appearing below T = 25°C was detected by powder X-ray diffraction. To gain a more complete understanding of the crystal chemistry of CAC, this new phase was synthesised in pure form and its crystal structure solved from powder X-ray diffraction data. The crystal data are a = 8.3312(4), b = 11.2670(5), c = 5.661(2)Å, Pna21 (No. 33) and the resulting chemical formula is KAl(CO3)(OH)2.H2O with Z = 4. From the refined positions of the non-hydrogen atoms and from the balance of bond valences, the chemical nature of the O atoms could be univocally established [Rwp= 0.132, RB = 0.056, Chi2 = 1.15]. The unit cell of this hydrated compound contains two carboaluminate chains formed by the basic unit [Al2(OH)4(CO3)2]2− propagating along the c axis and being stabilised by a complex H-bond network. The arrangement of these chains is similar to that found in dawsonite, so that the atomic positions of K+ and Na+ in both structures are also similar. As confirmed by the X-ray diffraction study of a one-year-old sample, hydrated potassium dawsonite slowly transforms into the non-hydrated form at room temperature. Due to the more favourable formation conditions, i.e. lower temperature and higher humidity, this hydrated phase could be eventually found in CAC pastes or mortars.