Growth factor receptors are activated through dimerization by the binding of their ligands and play pivotal roles in normal cell function. However, the aberrant activity of the receptors has been associated with cancer malignancy. One of the main causes of the aberrant receptor activation is the overexpression of receptors and the resultant formation of unliganded receptor dimers, which can be activated in the absence of external ligand molecules. Thus, the unliganded receptor dimer is a promising target to inhibit aberrant signaling in cancer. Here, we report an aptamer that specifically binds to fibroblast growth factor receptor 2b and inhibits the aberrant receptor activation and signaling. Our investigation suggests that this aptamer inhibits the formation of the receptor dimer occurring in the absence of external ligand molecules. This work presents a new inhibitory function of aptamers and the possibility of oligonucleotide-based therapeutics for cancer.