The creation of ultracold molecules is currently limited to diatomic species. In this letter we present a theoretical description of the photoassociation of ultracold atoms and molecules to create ultracold excited triatomic molecules, thus being a novel example of light-assisted ultracold chemical reaction. The calculation of the photoassociation rate of ultracold Cs atoms with ultracold Cs2 molecules in their rovibrational ground state is reported, based on the solution of the quantum dynamics involving the atom-molecule long-range interactions, and assuming a model potential for the short-range physics. The rate for the formation of excited Cs3 molecules is predicted to be comparable with currently observed atom-atom photoassociation rates. We formulate an experimental proposal to observe this process relying on the available techniques of optical lattices and standard photoassociation spectroscopy. Ultracold dilute gases at temperatures much lower than 1 mK offer new opportunities for the study of elementary reactive processes between atoms and molecules. As emphasized in seminal review articles [1, 2] this so-called ultracold chemistry is freed from averaging on large velocity distributions. Thus in this regime the quantum nature of the processes can be accessed, like the presence of resonances, or the sensitivity to long-range interactions which can induce anisotropic arrangements prior to the reaction. One of the most advanced experiments in this direction is undoubtedly under progress in JILA at Boulder, where quantum threshold collisions induced by tunneling through a centrifugal barrier between fermionic ultracold KRb molecules have been detected [3, 4]. Many experiments demonstrated that conditions suitable for inducing chemical reactions between ultracold ground state alkali-metal atoms and molecules tightly bound in their electronic ground state are reachable. Ultracold collisions between Cs atoms and Cs 2 [5, 6] or LiCs [7] molecules, of Rb and Cs atoms with RbCs molecules [8], or of K and Rb atoms with KRb molecules [9] have been detected through atom trap losses. Three-body recombination in a Rb quantum gas has been characterized [10], and features associated to universal N-body (up to N = 5) resonances induced by collisions between atoms and weakly-bound molecules have been observed in quantum degenerate gases [11–16]. However the deep understanding of the collisional dynamics is still to come as none of these achievements were able to characterize the nature and the state distribution of the final products [17, 18]. The heavy mass of alkali-metal species leads to a huge density of resonant states related to the numerous rovibrational levels accessible during an atom/molecule or a molecule/molecule collision , preventing them to be individually characterized. Models connecting the standard treatment of long-range interactions between particles to a statistical description of the resonances mostly resulting from short-range couplings have been developed [18–21]. The main quantita