Conventional methods to prepare hierarchical zeolites depend upon the use of organic structure-directing agents and often require multiple synthesis steps with limited product yield and Bronsted acid concentration. Here, it is shown that the use of MEL- or MFI-type zeolites as crystalline seeds induces the spontaneous formation of self-pillared pentasil zeolites, thus avoiding the use of any organic or branching template for the crystallization of these hierarchical structures. The mechanism of formation is evaluated by time-resolved electron microscopy to provide evidence for the heterogeneous nucleation and growth of sequentially branched nanosheets from amorphous precursors. The resulting hierarchical zeolites have large external surface area and high percentages of external acid sites, which markedly improves their catalytic performance in the Friedel-Crafts alkylation and methanol to hydrocarbons reactions. These findings highlight a facile, commercially viable synthesis method to reduce mass-transport limitations and improve the performance of zeolite catalysts.