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Calcineurin B-like (CBL) proteins are reported to play significant roles in plant development and ion-transport regulation. Potassium shortages are a serious problem in banana cultivation. However, to date, the members of the banana <i>CBL</i> gene family, and their function in regulating potassium stress, remain unclear. In this study, 11 <i>CBL</i> genes were identified from the banana genome and grouped into four groups (Group I–IV) based on their phylogenetic relationships. The genomic features of these <i>MaCBL</i> genes were analyzed, focusing on their gene structures, standpat motifs, chromosomal distributions, and evolutionary history. Expression pattern analysis revealed that the <i>MaCBLs</i> were function-specific. Further qRT-PCR analysis indicated that the presence of <i>MaCBL2</i> was indeed a response to potassium deficiency stress. The <i>MaCBL2</i> gene was cloned, and sequence alignment indicated that it contained four elongation factor hand (EF-hand) domains, the conserved N-terminal myristoylation domain “MGCXXS/K(T)” and the “FPSF” motif. Subcellular location analysis showed that MaCBL2 was located in the plasma membrane, nucleus and cytoplasm. The overexpression of <i>MaCBL2</i> could restore the growth of the yeast mutant R5421 on a K⁺-deficient medium. The overexpression of MaCBL2 could promote the root length of transgenic seedlings on K⁺-deficient medium. These findings indicate that MaCBL2 was, in our study, the key gene of the CBL family in responding to potassium deficiency in bananas. Our discoveries have established a considerable basis for the further study and application of <i>MaCBL</i> genes.