Density functional theory calculations were carried out to characterize a series of transition-metal-doped aluminum hydrides, forming TMAl(n)H(2n) and TMAl(n)H(2n+1) (TM = Sc, Ti, V; n = 3,4), in either charged or neutral form. A new electron-counting rule for these clusters was formulated as PSEN (paired skeleton electron number) = 4n, which can characterize both closed-shell and open-shell clusters. On the basis of this electron-counting rule, the superatomic clusters such as TiAl(4)H(9) and TiAl(3)H(6) were identified and can be used to assemble supramolecular structures. Electronic structure analysis showed that three-centered TM-H-Al bonds largely contributed to the structural stability. Also, the spin state of a wide range of clusters in their ground state can be predicted by the electron-counting rule.